Chapter 1:

The Basics of Paleoanthropological Moldmaking

An Introduction to Molds and Molding

Molds can be either natural or man-made, and both have histories stretching far back into prehistory. Both share one important feature--each kind preserves an impression of an original object. In nature, these molds are often called "trace fossils," and they preserve the shape, size, and morphological features of such items as fish, leaves, footprints, and even raindrops, that have often long since decayed or been destroyed by natural processes. Natural molds are the simplest type of mold, containing only a whole or partial impression of an item.

Man-made molds were originally as simple as natural molds, containing partial impressions of objects such as decorative relief motifs for sprucing up pottery vessels, or carved depressions in wood or soap stone used for making ritual items such as Egyptian shabtis and terracotta figurines. Over the past two millennia (especially during two periods: 1. around the time of the northern Italian Renaissance, and 2. during the last century), the sophistication of man-made molds has greatly increased in many ways--molds can now be made of multiple parts, and of very flexible materials (so much so that the molds can often be easily turned inside-out), they can incorporate interlocking self-aligning registration tabs, they can be made of materials that can capture microscopic detail, they can incorporate written information about the specimen, and they can incorporate features that allow the mold to be used for mass-producing casts.

In this manual, only man-made molds will be discussed. For simplicity's sake, man-made molds will be referred to simply as molds. Furthermore, since this is intended to be an introduction to molding for paleoanthropologists, the term "mold," as used here, will refer only to the sorts of elastomer-based and plaster- or plastic-encased molds made and used by paleoanthropologists, and not to any of the many other types specialized molds in use today, such as metallic injection molds used to mass produce plastics, ceramic molds used to form molten gold into ingots and bars, plaster and gauze molds used to mold living people or to make death masks, sand molds used to make some candles or to form certain cast metal items, or the lost-wax molds used to make jewelry.

For purposes of this manual, a mold is a scientific tool -- an entity which contains the impression of an original fossil (or bone) specimen from which copies of the specimen can be made for research, teaching, and display. By its nature, the impression contained within a mold will be a 'negative' or a 'reversed' impression of an original specimen. By introducing a compound such as a thick plaster slurry or unpolymerized plastic resins to the impression, and then closing the mold and allowing the compound to harden inside the impression, a replica, or cast, of the original specimen can be obtained which accurately reproduces shape, size and surface detail.

In a very basic sense, paleoanthropological molding can simply be preparing a molding elastomer, such as silicone rubber, pouring it over a specimen, and letting it polymerize and harden. Of course, there are numerous drawbacks to this method, especially on complex specimens such as vertebrate fossils, which often have delicate and complex morpologies. The drawbacks of this simple approach can fortunately be eliminated through the use of more refined techniques.

The molds you are likely to make as a paleoanthropologist may be as simple as one piece of cured elastomer, or they may more complicated affairs which may have multiple pieces of elastomer, incorporating numerous rigid blocks, all keyed and held together by a complex system of inner- and outer tabs, and surrounded by a multi-piece jacket which is in turn held together with bolts incorporated into the jacket pieces. The complexity and fragility of the specimen, combined with the level of completeness desired in the cast, will determine the complexity of the mold. It is likely that the vast majority of molds that you will make and encounter will be simple, two-part molds with two jackets and no blocks. It is this latter, most common type of mold that you will learn to make in this chapter.

Preparing your specimen for molding

In this manual I will not deal with the cleaning, removal of matrix, or surface preparation done on an original fossil specimen between the time it is discovered and the time it is fully described (and commonly referred to by paleontologists as "fossil prep" or "cleaning"), since that subject would require a manual of its own. The preparation I will be refering to here is the molding preparation which is done to the specimen after its initial cleaning in order to enable it to withstand the molding process.

Molding preparation is generally of two types, structural and superficial. Structural preparation includes filling in and buttressing cracks, brain cases, foramina, and other deep features and crevices on the surface of the fossil which are not desired in the replicas or which would constitute a threat to the specimen during the molding process. For instance, there may be a foramen which has a small external opening and which widens internally. If the uncured elastomer were allowed to flow into the foramen and cure, that portion of the mold would be difficult to remove without damaging either the specimen or the mold. In most cases, the internal morphology of the foramen is insignificant, and so the deeper portions of the foramen can be plugged, thus avoiding potential damage while still retaining the external morphology of the foramen. This type of preparation is accomplished with a material such as Plastilina, chosen for its sculptability, adhesion, and easy removal from the specimen.

The Plastilina can be added in small amounts, smoothed together and sculpted to fill most features, and can be fully removed from the specimen once its utility has passed. Although it can be fully removed, the Plastilina does contain oil which may penetrate the specimen, especially if the specimen is of a porous material such as unsealed plaster. These oils may permanently remain in, and possibly discolor, the specimen. If a porous material is to be worked on, and the possible penetration of small amounts of oils would be undesirable, it is recommended to seal the surface first with a material such as Glyptal® (a cellulose cement widely used in paleontology).

For large openings such as brain cases, where endocranial morphology is not desired on the finished cast, it is often impractical to use such large amounts of dense, heavy preparation materials, such as Plastilina. In these cases, materials such as crumpled toilet paper, plastic bags, or medical cotton batting may be used. The bulk of the opening can be stuffed with these lightweight, easily removable fillers and then capped with a sturdy plug of Plastilina or another similar material. The plug should form an even, clean line where it meets the specimen and should not be smeared over the surface of the specimen, thus obscuring detail. Effort should be made to ensure the plug is clearly demarcated from the specimen. This can be done with grooves or scoring, but often simply depressing the level of the plug a few millimeters below the natural surface of the specimen, leaving the plug smooth and featureless and having the plug meet the fossil at the unnatural angle of 90° will be sufficient. The filler and plug should be secure inside the specimen and deep enough in the opening (usually 2 to 5 mm) to allow the detail of the edges of the feature to be clearly seen.

Large cracks and deep crevices may also need to be filled in in their deeper portions, but as with the larger openings, not so much as to obscure the exact location and extent of the feature. When filling a crack or crevice, try to leave the top 1 to 2 mm unfilled. Many small cracks, as well as many large but shallowly sloping cracks or fossae will not need to be filled. Teeth alveoli will sometimes also need to be filled in their deeper portions.

The second type of molding preparation, superficial (or surface) preparation, refers to specimen-wide treatments to protect the specimen's surface during molding, while incurring no noticeable loss of surface detail. It also serves to remove any surface contaminants which may inhibit the molding compound from fully curing. Depending on the composition of the specimen and how it has been preserved, different methods and materials may be used. Non-fossilized bone may be crumbly, friable, or poorly preserved. In such cases, the specimen may need to be impregnated with a fully removable hardening solution such as Glyptal® to prevent the bone from exfoliating when molding with some of the higher fidelity (and therefore, more intrusive and adhesive) molding products. In other specimens, such as fossils with finely preserved detail, a fine powder such as talcum powder may need to be dusted over the specimen to prevent the high-fidelity molding rubbers from penetrating the innumerable microscopic depressions, thereby making removal of the specimen without damage difficult.

The molding and preparation history of the specimen should be carefully reviewed to make sure that substances capable of inhibiting the molding material either are not present, have been completely removed from the specimen, or have been completely sealed over with Glyptal®. Mold material incompatibilities are given in Appendix F.

Finally, as good molds are capable of reproducing microscopic detail, the specimen (excluding areas purposely plugged or sealed) should be free of all debris, including Plastilina, wax, adhesives, sealants, rubber, and plaster.

Determining the location of the part line

The flashline is the line that forms around the cast where the mold pieces come together at their part lines. The flashline on a cast poured from a two-piece mold is seen as a narrow band of elevated plaster that completely encircles the cast. The flashline is often one of the most obvious clues that the cast is indeed a replica. While it is usually not the highest priority for a molding and casting technician to make the flashline "disappear" (although see Chapter 2 for a discussion of this), it is important to make the flashline as inconspicuous as possible. Although there are methods for erasing the flashline once present on the cast, which will be discussed later, it is far better to properly locate the part line on the mold so as to minimize the impact of future flashlines on the esthetic and research value of the cast. For example, a researcher may be interested in studying the presence and extent of muscle attachments on a particular bone. If, in one case, the molding technician locates the part line along the small ridge of a muscle attachment, the future flashline will be along the ridge, confusing the exact extent of the ridge by adding to it both in height (which is difficult to sculpt down and remove exactly on an irregular surface) and in width (flashlines, no matter how thin, all have some thickness). If, on the other hand, the technician locates the part line slightly above or below the ridge, the ridge can be reproduced much more accurately.

Following are some guidelines for appropriately locating the part line on a specimen to be molded in a two-piece mold. Use these tips to locate a hypothetical 'best' part line for the specimen to be molded, and keep its location in mind while going on to the next step, setting up the specimen.

* The part line should divide the specimen into roughly equal halves.
* Locate the part line so as to minimize overhangs on the specimen.
* Avoid positioning the part line on important anatomical morphology.
* Avoid placing the part line on tooth crowns.
* Visualize possible difficulties in pulling casts and try to minimize these.
* Avoid needlessly complex part lines.
* Try to have the plane of the part line roughly parallel to the plane of the set-up block.

Setting up the specimen for molding

The set-up block is simply a working platform for the specimen to be molded upon. It is generally sculpted from a modeling clay such as Plastilina, although other materials (such as cardboard, Plexiglas, and wood) are often employed. The set-up block is sculpted to surround the specimen, exposing those portions to be molded initially and masking those areas to be molded later. Furthermore, features are included on the set-up block which will be molded along with the specimen. These features are things such as tab forms, which will aid in proper alignment of the mold pieces, and a mold label, which will aid in the correct identification of the specimen.

To make a basic set-up block for a single, simple specimen, begin with a slab of Plastilina about 2-3 cm thick and several centimeters longer and wider than the specimen. Set the specimen on the center of the block with the plane of the intended part line roughly parallel to the plane of the Plastilina block. Carefully, and preferably with a soft or blunt tool, such as a dental tool, outline the specimen on the block. To avoid damaging the specimen, do not drag the tool along the specimen or scratch the specimen while tracing this outline. Remove the specimen and carve out the inside of the outline to a level deeper than the height of the specimen to the hypothetical part line. No great care need be taken to make the depression correspond to the exact contours of the specimen. Set the specimen into the depression cut for it and compare the plane of the set-up block to the plane of the intended part line. The two planes should be made coplanar by removing or adding pieces of Plastilina to the carved depression.

Once the specimen is properly positioned, gaps between the specimen and the set-up block will have to be filled. If there is a gap of only one or two millimeters, a blunt, flat-bladed dental tool can be used to sculpt and extend the block over to the specimen. If a larger gap exists, roll some Plastilina into a long, thin strip and place it into the gap. Next, use the same dental tool to smooth the strip up to the specimen as before. The Plastilina, where it meets the specimen, should be level and should cleanly meet the specimen without any smearing onto the surface of the specimen. Sculpt the block up to the specimen around its entire circumference. Use long strokes with a flat-bladed dental tool all along the part line to ensure the part line is smooth up to the specimen and out onto the set-up block. Make sure that no excess Plastilina adheres to the specimen above the proposed flashline. Additionally, be sure that when working with the set-up block your hands are clean and free of Plastilina before touching the specimen. Plastilina on the specimen obscures minute details and leaves Plastilina fingerprints that may show up on the final cast. Once the specimen is set into the set-up block, smooth the surface of the block to remove any bumps or gouges. Your finger will work nearly as well as a metal sculpting tool and will certainly be faster. The smoother this is done, the more easily the excess plaster will flow out in the casting process, which will ultimately result in a thinner flashline and a better cast.

Adding tab forms and a mold label

Tab forms, which are sculpted into or onto the set-up block to define the shape and location of the future tabs, will now need to be made. These can either be pre-formed, truncated pyramids made of plaster or sculpted of Plastilina, or depressions sculpted or impressed directly into the set-up block. Both methods work equally well, but impressing the tab forms into the set-up block with an appropriately sized and shaped die is a much faster and neater method; therefore it will be the method discussed here. Alternative methods will be discussed in Chapter 2.

Enough tabs will have to be made to ensure good registration and a tight fit between the two halves of the mold, as well as leaving sufficient areas for the excess plaster to drain out of the mold. Each mold is unique, but I find that 3 to 5 tabs for a 10 cm x 15 cm mold is sufficient. Large, complex molds may need many more. Generally, tab forms will only need to be placed towards the edge of the set-up block. Tab forms placed too near the center of the mold will result in tabs which may seem to work well, but which may not properly interlock when casting, thus resulting in a distorted cast with a thick flashline.

Before the tab forms are placed into their positions, the final size of the set-up block will have to be determined, or at least estimated. Determine where the tab forms will be placed and the location of the mold label (mentioned below). Leave about a 1 cm margin between any of the features of the set-up block and the intended edge of the trimmed block. Use a straight edge to lightly impress lines to mark the intended future edge of the set-up block. The block will not be trimmed until later, but these lines will aid in placement of the tab forms and the mold label.

To make impressed tabs, find or make an appropriate die (I use the butt-end of an older model Sharpie® pen) and press it into the set-up block in the desired positions. I find the tabs are cleaner if I twist the die slightly while pressing and removing it. Often, the Plastilina will bulge upwards around the depressed tab form. This can be removed by trimming with a dental tool or small knife and then re-impressing the die into the depression to clean it up.

Future casting and general organization and curation of the molds will be greatly facilitated if a specimen label or other unique identification is incorporated permanently into the mold. One easy method makes use of a Dymo® or other similar label making machine. Using the Dymo® , type out a complete but concise unique identifying label which will easily fit on the set-up block between the tab forms. Turn the label face down and impress the letters into the set-up block. If done properly, a clear reversed impression of the label will remain on the set-up block and the label itself can be discarded. If done incorrectly, the impression of the label can be erased with your finger and re-done.

Finally, the set-up block will have to be trimmed down to neaten the edges and to reduce the set-up block to the minimum necessary size. With the lines that were lightly impressed into the set-up block earlier as guides, use a bladed putty knife or a kitchen knife to trim the set-up block neatly to a vertical or slightly undercutting edge. The angle at which the set-up block is cut will be important later, when we begin to build the containment walls around the set-up block.

Preparing the molding rubber

After the specimen is completely set-up, the block is smooth, and the specimen is clean and free of debris, the process of making the impression begins. In this example, we will be using Dow Corning Silastic® E RTV (a silicone rubber) and its catalyst as the elastomer. The Silastic® E has the look and consistency of marshmallow topping, and its catalyst has the look and consistency of mineral oil.

To ensure a better release of the molding rubber from the specimen, it is best to coat the specimen liberally with talcum powder, taking care to coat all areas of the specimen. With a fine stream of pressurized air, blow the talcum powder off of the specimen. Done properly, this leaves a fine dusting of talcum powder adhering to the specimen, visible only as a matte sheen on the surface.

With an accurate scale, weigh out a paper cup (or preferably reusable plastic cup) with a capacity of approximately five times that of the molding rubber needed. Let us assume the cup weighs 18 g. This is the tare weight of the cup. Add to this weight on the scale an additional 50 g. This should be enough Silastic® E rubber for a first coat on any small to moderately sized mold. The scale with the cup on it should now read 68 gm, and will be out of balance. Add the molding rubber to the cup on the pre-adjusted scale until the beam balances. If there is any mismeasurement, correct by adding or subtracting Silastic® E from the cup until the beam is balanced.

Now add to the weight on the scale 10% of the weight of the Silastic® E. If you weighed out 50 g of Silastic® E, the scale with the cup and the Silastic® E on it should now read 73 g. That is, 18 g (for the cup) + 50 gm (for the Silastic® E) + 5 g (for the Silastic® E catalyst). Add the liquid catalyst to the cup slowly and carefully until the beam balances nearly exactly. Care is warranted at this point because corrections for adding too much catalyst cannot be easily made.

With a wooden tongue depressor, or preferably a metal artist's spatula, completely mix the Silastic® E and catalyst in the cup for approximately 2 to 3 minutes to ensure complete homogenization of the rubber. Be sure to get into the corners and the bottom of the cup. Failure to completely mix the rubber could result in incomplete curing of the rubber in areas, ruining the mold and making a mess of the specimen. Do not worry about hurrying, as the Silastic® E will stay workable for a couple of hours. Once the rubber is mixed, throw the wooden tongue depressor away, or wipe the rubber off the metal spatula so that it can be used again.

Once the Silastic® E is fully mixed, it will need to be de-aired to remove air added during mixing and air which is naturally present in the rubber when stored at normal atmospheric pressure. The de-airing will be accomplished inside a vacuum jar attached via a vacuum hose to a vacuum pump. The vacuum pump should be capable of drawing a vacuum of 28 to 29 inches of mercury. Place the fully mixed Silastic® E container inside a sealable vacuum jar. Close the jar and any outside valves, and attach the vacuum pump hose and open its valve. Turn the vacuum on and let it achieve a vacuum of 28 to 29 inches. Under such a vacuum, the Silastic® E will froth and expand to 4 or 5 times its original volume; hence the reason for choosing such an oversized container when weighing the Silastic® E. Once the Silastic® has achieved its full height, it will quickly collapse back onto itself and will recede to its original volume. After this has happened, let the vacuum run an additional 1 to 2 minutes.

When the time has elapsed, close the valve between the vacuum pump and the vacuum jar. Open the valve on the vacuum pump and let it run in this open position for 1 to 2 minutes before shutting it off. This will increase the life of the pump by allowing the pump to purge itself of oil fumes while operating under a minimal load. Open the valve on the vacuum jar to re-admit air to the jar, or detach the pump hose and let the air back in via that route. Once the pressures have been equalized, which will take about 10 seconds, open the vacuum jar and remove the Silastic® E container. The rubber is now ready to be applied to the specimen.

Applying the detail coat

With a wooden tongue depressor or a metal spatula, scoop up some Silastic® E from the container and let it dribble over the specimen. This should be done without ever touching the spatula to the specimen. Complete coverage of the specimen and elimination of entrapped air bubbles are not important at this stage; the point is merely to lay some rubber on the specimen.

For properly applying the rubber layers, a compressed air source and an air gun will be needed (further discussion of these can be found in Chapter 7). Turn the valve on the air gun to get a small amount of air flowing in a fine stream from the nozzle. Use this fine stream to push the Silastic® E over the entire specimen. Add additional rubber when needed, always remembering to avoid touching the specimen or the set-up block with the spatula. For getting Silastic® E into the tiniest and deepest features, a syringe with a fine, soft plastic tip may be needed. Monoject® makes a 12 cc syringe with a tapered, curved tip which works well for this purpose. Remove the plunger and add a small amount of rubber to the syringe. The syringe can then be used to inject small amounts of rubber into otherwise difficult to access areas.

Once the specimen (but not necessarily the whole set-up block) is completely covered with a thin coat of Silastic® E, use the stream of air to work the Silastic® E into all the fine cracks and crevices on the specimen. Several attempts may have to be made on some of the most stubborn of features, but with patience this can be easily accomplished.

After the entire exposed surface of the specimen is covered with Silastic® E, the task of eliminating entrapped air begins. Any air trapped in the rubber against the specimen in this first coat will result in bubbles on the impression surface of the mold. These in turn will result in beads on the cast of the specimen. These will have to be removed, and will obscure the detail that would have been present on the specimen in the area of the bead.

To eliminate entrapped air in this first coat, simply eliminate any excess rubber that the bubbles could become entrapped in. This is done by turning the valve on the air gun to slightly increase the volume (and thereby, the pressure) of the air flowing from the gun. Use the air stream to blow the rubber quite thin on the specimen. Start from the top of the specimen and work down, using the air stream to push any excess rubber off the specimen and onto the set-up block. The rubber should be blown thin enough to see through over the entire specimen. Several passes with the air gun may need to be made. Do not worry either about blowing the Silastic® E too thin or drying the Silastic® E by blowing too much air on it; neither will happen. When blowing the excess rubber off the specimen, pay special attention to thin cracks and deep features, and to the area around the part line. These are the areas most likely to retain rubber and entrap bubbles.

Once the rubber is blown thin and evenly over the entire specimen, make sure all areas which may entrap air bubbles have been checked and cleared of excess rubber, and that all excess rubber sits out on the set-up block. Set the mold-in-progress aside and check it about 30 minutes later for areas where rubber may have flowed and accumulated and for bubbles which may have formed anywhere on the specimen. At this point, the application of the first coat is finished.

Building the containment walls

Immediately after the first coat has been applied and checked and before the rubber sets-up, containment walls must be built around the mold to keep future layers of rubber and plaster from flowing away from the set-up block. The walls should be erected around the set-up block while the Silastic® E is still workable. The walls also serve as a form to give the sides of the future mold their shape. If the containment walls are done neatly, the mold will be smooth and even and easy to clean after casting.

Containment walls are frequently sculpted of Plastilina slabs or constructed of plexiglass acrylic sheets. The use of Plastilina will be discussed here, with the use of plexiglass being discussed in Chapter 2. A slab of Plastilina should be cut which is about 2 cm longer than each side of the set-up block, about 2 cm higher than the highest point of the specimen (measured from the bottom of the set-up block), and about 2 cm thick. Weld the walls to the set-up board, and if necessary, the walls to each other, by pressing a long, thin strip of Plastilina firmly into any gaps to make the set-up block and the walls a solid unit.

The joints between the walls are sealed with Plastilina to prevent molding materials from leaking out on future layers. As the molding materials being used, both the Silastic® E and the jacketing plaster, are very viscous, the joints need not be perfectly watertight; the materials will not be able to flow from the smallest cracks.

After the containment walls are constructed, set the mold aside in a clean, dust free area to cure for at least 24 hours at room temperature (roughly 24 hours at about 25° C, or 77° F) before going on to the next step. The clean, dust free environment will help to ensure that contaminants which may inhibit the Silastic® E are not present.

Applying the intermediate coats

After sufficient time has passed for the first coat of the Silastic® E to cure , work on the next coat can begin. For the second coat, as with the first, weigh, mix and de-air the Silastic® E. The second coat will be applied similarly to the first, with the exception that much of the fine detail has been covered by the detail coat and will present much less of a problem. Nevertheless, you should pay attention to ensuring that no air bubbles are entrapped within the rubber. With the second coat, the whole set-up block gets covered with rubber out to the containment walls. The second layer will be thin, but not as thin as the first. The rubber can be allowed to flow up onto the walls to a height of about 1 cm. Rubber that adheres higher can be blown back down or will be removed later in the molding process. Though not critical, pay attention to the corners between the set-up block and the walls to ensure that no bubbles become entrapped there. After the second coat is finished, set it aside and check it again after about 30 minutes, looking for any bubbles or places where they may be entrapped. After checking, set the mold aside in a clean area to cure for at least 24 hours at room temperature before going on to work on the next step.

For the next coat, a thickness-building coat, weigh, mix and de-air the Silastic® E as before, with the exception that 2 to 3 times the volume of rubber should be used. Apply the rubber as before, with the exception that the rubber may be brushed on to the mold with light strokes, instead of being blown on. Take care not to displace or tear the fragile first two layers. The third coat can be much thicker than the first two, although the technician should still be aware of the potential of entrapped bubbles. As before, after the third coat is finished, set it aside and check it again after about 30 minutes, looking for any entrapped bubbles. After checking, set the mold aside in a clean, dust free area to cure for at least 24 hours at room temperature before continuing with the next step.

Applying a gauze reinforcement

Once the first few coats have been applied, the mold-in-progress should be sturdy enough to accept a gauze reinforcement layer. Weigh, mix and de-air the Silastic® E as before, with the exception that twice as much rubber should be used as was used in the last layer (roughly 5 times as much as the first two coats). Be sure that the container used for mixing and de-airing is sufficiently large (use a cup with 5 times the volume of the Silastic® E needed) to allow the rubber to expand without spilling over. Once the rubber is ready, brush on a 1 mm layer over the entire mold.

For this coat, a layer of gauze will be incorporated into the mold to give it additional tear resistance and ensure nominal thickness in the thinnest portions of the mold. The gauze used can vary, although Johnson & Johnson Kling® gauze bandages work well because they can stretch and adapt to odd shapes.

Take a clean, flat surface about 20 cm by 30 cm, such as plexiglass or masonite board, and brush on a 1 mm layer over a surface about 10 cm by 20 cm. Onto this patch, lay a piece of gauze about 5 cm by 10 cm. Pat it firmly into the Silastic® E on the board and then brush on additional rubber until the gauze is saturated and free of air bubbles. With gloved hands, pick up this rubber-saturated gauze and cut it with scissors into workable pieces that can be laid onto the pre-coated mold surface.

Take one of those pieces and lay it onto the surface of the mold. Using a clean, disposable acid brush tamp the gauze down firmly so that it conforms to all the contours of the mold section it overlays. Repeat with other pieces of saturated gauze, overlapping each piece slightly with earlier placed pieces until the whole mold surface is completely covered. While you should lay gauze all the way out to the walls, do not get so close to the walls that the gauze actually touches a wall. In most areas of the mold, fairly large, rectangular pieces of gauze may be used, but in some complex areas, much smaller, odd-shaped pieces should be employed. These pieces may additionally have to be slit along edges to bend around steep angles. When the gauze coat is complete, carefully tamp the entire surface with the acid brush to check for any voids or air bubbles. Once again, check after 30 minutes, and then set aside in a clean area to cure for at least 24 hours at room temperature before going on to the last step.

Applying the final coats

Once the gauze coat is cured, weigh, mix, and de-air an amount of Silastic® E roughly equal to twice the amount used for the first two coats. Brush this coat over the gauze to completely cover any traces of the gauze material. Bubbles in this coat are of no great concern, but nevertheless, blow over the surface after finishing and 30 minutes later, if convenient. As always, set the mold aside in a clean area to cure for 24 hours at room temperature.

The final rubber coat on this side of the mold involves simply the attachment of pre-cut Silastic® E tabs, used between the mold and the plaster jacket. Weigh, mix, and de-air the Silastic® E as always. Only a small amount of rubber is needed, roughly equal to the amount used in each of the first two coats. Use some of the Silastic® E to cover up any remaining areas where the gauze fabric may be showing through.

Silastic® E tabs are made from left-over cured Silastic® E rubber remaining in the bottom of mixing cups to a depth of about 4 to 12 mm. Pull the dried rubber out of the bottom of the cup and trim off the edges with scissors. Then place the rubber plug on a flat, hard cutting surface such as glass. With a sharp blade, such as an X-Acto® knife, cut the plug into 1 cm wide strips. When cutting, have the blade at a slight angle and flip the plug over after each cut. Done correctly, you should end up with a half dozen or so long strips of cured Silastic® E that are wider at their bases than at their tops. Use these strips as blanks from which to cut small truncated pyramidal shapes. You'll find the cuts come out much cleaner if done in one long straight cut applied with quite a bit of pressure rather than a back-and-forth saw-like cut, which results in jagged cuts.

Apply a generous amount of Silastic® E to the bottom of a tab and position the tab on the mold surface. The Silastic® E will cure and will bond the tab permanently to the mold. Each mold is unique, but usually 4 to 6 tabs are sufficient for most small to moderate-sized molds. Once again, set the mold aside in a clean area and let it cure for 24 hours at room temperature.

Forming a plaster jacket

When used for making casts, a mold has to have two very important, but contradicting, features. It must be rigid to prevent any distortion while the casting liquid is curing or hardening within. After the cast has cured, however, a flexible mold is highly desirable so that the cast can easily be removed from the mold without damage to either. Using the Silastic® E for the main portion of the mold satisfies the flexibility requirement. The Silastic® , however, while fairly rigid in thicker sections, is not always enough to ensure distortion-free casting. This is where the plaster jacket, or mother mold, comes into play. A form-fitting plaster jacket around the rubber portion of the mold adds the necessary stability, yet after casting it can be removed to give the mold back its flexibility.

Once the mold has cured for at least 24 hours, trim those fingers of Silastic® E adhering to the walls above the level of the mold. In order for the plaster jacket to separate more easily from the rubber , dust the rubber surfaces liberally with talcum powder.

For general jacketing, Hydrocal® B-ll gypsum cement makes a good medium. This plaster is gray, sculptable, and hardens in about an hour. Add a little less than 3 parts plaster by weight to 1 part tap water in a reusable (plastic or waxed paper) cup and mix together with a tongue depressor or an artists' spatula, trying to eliminate the largest lumps. Air bubbles are of no great concern at this stage, but tapping the container on the counter top a couple of times will help to raise the largest of the air bubbles.

With your hand, add small amounts of the plaster inside the containment walls. Jiggling the plaster once it's in the mold or placing the mold on a vibrating platform will help settle it around the features of the mold. Once the features of the mold are submerged in the plaster, add enough to ensure at least 5 mm of plaster above the highest feature on the mold. After the plaster is poured on the mold, set the mold aside to harden for about one hour. Towards the end of this hour, the plaster will gel and semi-harden, allowing the technician to work the plaster before it fully hardens. In this state, the walls can be removed and the plaster worked with wet hands to eliminate any sharp corners or edges. When the plaster has thoroughly set, let the jacket dry and release its steam for at least a few hours before going on to the next step.

Preparing the second side

Once the jacket is dry, you can begin work on the second side of the mold by using a putty knife to separate the set-up block and the rest of the mold from the set-up board. Clean the board of any adhering Plastilina and set it aside to use when assembling the containment walls around the second side. Flip the mold over so that the plaster jacket is on the bottom and the set-up block is on top.

Before proceeding with molding, the set-up block will have to be completely and carefully removed from the rest of the mold. Begin by using large, flat-bladed dental tools and small metal spatulas to remove pieces of the set-up block while being careful not to scratch or dislodge the specimen, which is now half embedded in the mold. Continue in this fashion until the majority of the Plastilina set-up block is removed and only small adhering pieces remain in places such as along the part line and in the fine morphology of the specimen.

Using a fine dental pick, or the tip of a needle, remove all of the smaller pieces of Plastilina. Continue until you have reached a point where all that remains on the specimen and the rubber portion of the mold are smears of Plastilina and pieces too small to remove with even fine tools. At this point a solvent, such as acetone, is the best for removing the smallest bits that remain. Place a small amount of acetone into a conical flask to keep the fumes to a minimum while allowing easy, open access to the solvent. Insert a dozen or so long-stemmed (6") cotton-tipped swabs into the top of the flask. With these pre-soaked swabs, you can remove any excess Plastilina from the surface of the Silastic® E or the specimen. Pay special attention to the area around the part line. When you are satisfied that all Plastilina has been removed from the mold, set the mold aside for a few minutes to allow any remaining acetone to evaporate.

Molding the second side

As with the first side, give the specimen a liberal dusting of talcum powder over all exposed surfaces. With a fine stream of pressurized air, blow the talcum powder off of the specimen. As before, this should leave a fine dusting of talcum powder adhering to the specimen, visible only as a matte sheen on the surface.

New layers of Silastic® E bond extremely well to older layers, as can be seen in the many layers of the first side of the mold. To avoid the layers of the second side from bonding with the first, thereby sealing the specimen inside a solid block of Silastic® E, an intra-mold separator which keeps the two sides separate will have to be applied.

An intra-mold separator is simply a thin material that forms a barrier across which the Silastic® E cannot bind. One which has proven to work very well, and which is recommended by Dow Corning, the manufacturers of Silastic® E, is a 5% solution of petroleum jelly dissolved in 95% 1.1.1 trichloroethane. Petroleum jelly alone does not work well as a barrier because it is too thick and irregular when applied undiluted. The trichloroethane serves to thin the petroleum jelly during application and evaporates immediately afterwards.

The separator solution should be made and used under a fume hood because of the volatile nature of the solvent. Measure out the 1.1.1 trichloroethane and place it in a sealable, solvent-proof bottle. Measure out the petroleum jelly by volume (there is no need to be overly accurate), and place it in a small pan in a double boiler arrangement. Gently heat the petroleum jelly until liquid, at which point it can be added to and easily mixed with the trichloroethane within a few seconds. Although the mixing can be done without heating the petroleum jelly, it will take much longer, perhaps 10-15 minutes.

To apply the intra-mold separator, use a small paint brush dipped in the 1.1.1 trichloroethane/petroleum jelly ("tri-vaso") mixture and carefully paint all exposed Silastic® E surfaces, including the flange, the tabs, the label, and the rubber sides of the mold. There is no need to apply anything to the plaster jacket, and no tri-vaso should be applied to the specimen. The addition of the petroleum jelly will obscure the details of the specimen. Once the rubber is completely covered with the separator, set the mold aside for a few minutes to let any remaining solvent evaporate before going on to the next step.

After the talcum powder and the intra-mold separator are applied, the formation of the second side of the mold can begin. In this example, and in general, the second side of the mold will be applied exactly as the first. Begin with a detail coat, then add some intermediate coats, a gauze coat, the final coats, the tabs, and finally the plaster jacket.

When the plaster jacket on the second side has hardened, the specimen can be extracted from the mold. Removing the specimen from the mold will be quite a bit more difficult than removing subsequent casts from that same mold. Because the Silastic® E is capable of reproducing very fine morphology, it will penetrate and closely adhere to all the fine pores, cracks, and foramina on the specimen. Additionally, as the Silastic® E softens certain types of glued joins, the specimen may be removed in several pieces, which will all have to be reassembled.

Begin by removing the two jackets from the mold and setting them aside. While holding the mold over a soft, clean surface, slowly break the seal between the two sides of rubber. Work slowly and from the point of least resistance, and proceed around the circumference of the mold until the flanges of the mold are separated all the way in to the specimen.

Once again working from the point of least resistance, gently begin pulling and working the rubber away from the sides of the specimen. Pressure may have to be exerted on the specimen through the rubber with one finger while pulling the rubber away from the sides of the specimen with the rest of the hand and using a free hand to gently pull on the specimen itself. Most specimens can be removed easily after gently working them out for a minute or two.

After the specimen is removed, check for any adhering pieces of the specimen remaining inside the mold impression. If the pieces are small, they can be left in the mold until you are ready to repair the specimen, so that it will be easy to locate their position on the specimen. Once the specimen is fully removed from the mold, reassemble the mold and set it aside for a few days to fully cure. Dow Corning recommends 7 days' time to achieve a final cure before producing any casts from the mold.

When the mold is fully cured, some final touches are needed to allow easier identification of the mold and to make cleaning the mold simpler after casting.

First, the plaster jackets should be sealed to harden the plaster and to make future removal of casting plaster and resins much easier. For this purpose, Glyptal® is most often used. Under a fume hood, use a [10%] solution of Glyptal® dissolved in [90%] acetone and a disposable acid brush to paint the Glyptal® solution over the entire surface of each jacket. Two or three coats can be applied until the Glyptal® no longer penetrates, but merely remains on the surface. Leave the jackets under the fume hood for about 15 minutes, or until the solvent has totally evaporated.

With an indelible marker, the specimen label can be written on the outside surface of the sealed jacket. Information such as a brief description of the specimen, or a species name may make future identification of this mold easier. The mold is now fully finished and ready to be used for the production of quality replicas. Basic casting techniques will be described in Chapter 3.

Before the molding process is complete, some work may have to be done on the specimen to alleviate any ill effects of the molding process.

As discussed previously, the molding compound used may have dissolved or softened glued joins on the specimen. These will have to be reglued or refortified. Glyptal® makes a superb adhesive for such joins, because of its complete removablility. To make a strong join, begin by coating all surfaces to be joined with [10%] Glyptal® thinned in acetone. While this thin Glyptal® is still wet, apply a dab of 100% Glyptal® to one of the surfaces being joined and re-articulate the fragments. Excess Glyptal® can be removed easily while wet with cotton swabs soaked in acetone. Set all joins aside to dry for several hours to a day.

The foramina, cracks, and other features that were previously filled with Plastilina will now have to be cleared of their protective plugs. The bulk of this can be done manually with dental picks or with a needle in a needle vise, but the fine pieces remaining can be removed by lightly rubbing with a cotton swab wetted with acetone. A thin Plastilina residue may also exist around the circumference of the specimen at the level of the part line. This can also be removed with an acetone-wetted swab.

ContentsPrefaceChap. 2Chap. 3Chap. 4Chap. 5Chap. 6Epilogue
GlossarySafetyMaterialsSuppliersBibliographyMat. Specs.