Chapter 3:

The Basics of Paleoanthropological Casting

For purposes of this manual, a cast is a replica of an original specimen which is made from a mold of the specimen. In a very basic sense, the cast is formed by mixing and adding a casting compound, such as plaster or any of several plastic resins, to the 'negative' impression in a mold. The mold is then reassembled, leaving the casting compound sealed inside the impression to harden. After sufficient time has passed, the solid cast can be removed from the mold. Flash, the thin sheet of material that hardens between mold pieces, can be removed, leaving only a trace line around the circumference of the cast, which is called a flashline.

As with molding, there are numerous drawbacks to this simplistic method, especially on complex specimens such as vertebrate fossils, which can be solved with more refined techniques. In addition, there are specialized techniques such as hollow casting, centrifugal casting, and impregnation techniques that may need to be applied. The techniques applied and materials used will depend on the nature of the specimen to be cast, as well as on the purposes and uses to which the cast will be put.

In this chapter, only the production of solid casts made of plaster from a two-part silicone rubber mold will be discussed. The numerous alternative methods and variations on those methods are discussed in chapter 4.

To make a cast, casting compounds are added directly to a specimen's impression in a mold, ideally closely penetrating the various and sometimes fine morphologies of that impression. Without preparation in the form of mold release agents and surfactants, or wetting agents, fine detail may be lost, and worse, the mold may get damaged. Mold release agents usually come in the form of oils and greases, sometimes in an aerosol form, and are generally needed only when casting in plastic, where exothermic and other chemical reactions may cause the molding compound to quickly degrade. Further discussion of casting in plastic and the use of mold release agents are discussed in chapter 4.

When casting in plaster, there is generally no need for a mold release agent. While many plasters are indeed quite exothermic, the compounds used are essentially inert, not reacting with or degrading the molding compounds. What is needed, however, is a wetting agent to ensure that the plaster, when initially added to the mold impression, penetrates and fills all the minute, fine features in the impression, thereby more precisely duplicating the morphology present in the original specimen.

A wetting agent is essentially a dilute soapy solution that is used to fill the mold and then is removed from the mold immediately prior to casting, leaving a wet film over the surfaces of the impression. This wet film acts as an interface between the dry silicone rubber mold and the liquid plaster. It serves as a surfactant, breaking down surface tension and making it easier for the plaster to flow along the rubber surfaces. The wetting agent works in the other direction as well, by eliminating the surface tension that would otherwise allow small air bubbles to remain in the fine features of the mold. The plaster slips into the mold more easily and entrapped air bubbles more easily slip out of the mold during the initial fill with the casting plaster.

A wetting agent which works quite well to this end is Permaflex® mold dressing. It is sold in a concentrated blue liquid form, which is then diluted for use. Although exact dilution is not necessary, the manufacturer recommends a ratio of 1 part Permaflex® to 48 parts water, by volume. Mixing in this ratio gives a sudsy, light blue solution. Use of a 250 ml graduated cylinder allows accurate measurement, while the construction of the cylinder makes dilution and mixing a single step when the Permaflex® is measured first. Once the Permaflex® is diluted, the solution can be added to a polyethylene wash bottle for easier, neater, and more precise application. The solution, if kept in a pure form, will last for up to several weeks; however, if used and reused for several molds during the course of the day, it will become contaminated by plaster and other debris and must be remixed daily.

Before applying the Permaflex®, the mold should be checked to ensure that it is cleaned and in proper condition for casting. Check the internal rubber surfaces first to make sure that there is no adhering plaster either in or on the tabs, on the flange, or inside the impression. Nothing should normally be inside the mold impression unless the last cast produced from this mold was defective and left broken portions inside the mold. The flange and tabs, however, are normally covered with flash after every casting, and must be fully cleaned each time. Any plaster left adhering to the flange or to the tabs will increase the thickness of the flashline on the next cast to be poured, thus lessening the research and aesthetic values of the cast. All the elements of the mold can be cleaned with blunt dental tools.

Next, the plaster jackets should be removed and the outer surfaces of the rubber checked. Normally when casting with plaster, and especially when using water to remove excess plaster from the mold before hardening, a thin plaster film builds up on the outer rubber surfaces. A dental tool can be used to scrape off any adhering film. While the jackets are off, the inner surfaces should be checked as well for any adhering plaster. If the jackets have been hardened and sealed with Glyptal®, removing the plaster will be an easy task. If, however, the jackets are unhardened, the casting plaster may bond to the jackets. Finally, the mold should be reassembled and checked to see that there is no adhering plaster or other compounds on the outside surfaces of the mold which may interfere with casting, or with the drainage of excess plaster from the mold. This being done, the mold should be reopened and set face up on the casting area countertop.

With the previously prepared wash bottle of Permaflex®, the wetting agent should be squirted over all surfaces of the specimen impression on the mold to be cast. The wetting agent should be left in the mold until after the next step of the casting process.

The next step in the casting process is the weighing and mixing of the casting plaster to form a plaster slurry. The plaster to be used in this example is a high quality, fine grained, buff colored dental stone called Coecal®. To increase its strength, hardness and chip resistance, it will be mixed with a hardening solution instead of water. Two good brands of hardener are WhipMix® gypsum hardener and Buffalo Stalite® Special Liquid Solution. Both are slightly milky looking, highly mineralized liquids.

It is recommended that from this point until the end of the casting process that a plastic apron and older clothes be worn, and that long hair be tied back out of the way. Liquid plaster tends to be sprayed onto clothes, skin, and hair during some of the procedures, where it can usually be removed without incident. Additionally, people with respiratory problems may wish to use a dust mask while working with the powdered plaster, and those with sensitive skin may choose to wear latex or vinyl examination gloves while dealing with the plaster slurry.

The amount of plaster needed for any particular cast is somewhat difficult to determine unless data sheets have been kept for the mold. The ability to accurately estimate the volume of the specimen's impression, and thereby calculate the necessary amount of plaster is a skill which is difficult at first, but which can be developed with time and practice. In the absence of either records or experience, other methods may be employed. If a good cast previously made from the same mold is available and does not seem to have any large internal air voids, the cast can be weighed and 125% to 150% of the weight of the cast used as an estimate of the amount of dry plaster needed to produce the cast. The extra 25% to 50%, as well as the uncalculated weight of the hardening solution, will offset the amounts that remain in the mixing bucket and pouring bowl, the amount lost when blowing on the detail coat, the amount of run off on high relief molds, and the amount overfilled to ensure a secure closure and a mold impression filled with plaster. Another method is to weigh the unfilled mold, then fill it with water and re-weigh it to determine the amount of water needed to fill the mold. By multiplying by 1.7, the weight of plaster needed can be determined, which can then be multiplied by 125% to 150%, as above.

Once the amount of dry plaster needed is determined, an accurate balance is used to tare a cup and weigh out the dry plaster in the cup. With another cup, a corresponding 33% of the hardening solution is weighed out. The measured cups are set aside for a moment while a check of the casting equipment is made.

Before mixing the plaster slurry, the mixing equipment should be checked to ensure that it is in proper working condition. The mixer that will be used in this example is a sturdy workhorse used in dental lab work and designed to be used with dental stones such as Coecal®. The mixer is called a Power Mixer and is manufactured by the WhipMix corporation. It consists of a low RPM, high torque motor which connects to a mixing paddle incorporated into the lids of various sized mixing buckets. Additionally, the mixer incorporates a vacuum pump capable of drawing a vacuum of more than 29 inches of mercury. This vacuum is connected to a gauge and to a vacuum line with a fitting that can be attached to the mixing bucketto remove entrapped air during mixing. The vacuum line is further equipped with a plaster filter to prevent plaster from entering the vacuum pump if it enters the vacuum line. All of the components of this mixing system need to be quickly checked before mixing.

Locate the appropriately sized WhipMix mixing bucket and make sure that the rim of the bucket, the corresponding area on the lid, and the o-ring around the lid are clean and in satisfactory condition. If any of these areas are not clean, use a blunt dental tool to clean them. The o-ring can be removed and cleaned under running water. Dried plaster adhering to any of these areas will contribute to a loss of vacuum later, which will result in a frothy slurry, and may result in a cast ruined by masses of micro-bubbles.

The oil level in the external oil reservoir on the Power Mixer should be checked. The vacuum line and the plaster filter should not be clogged with plaster and the small o-ring on the end of the vacuum line should be clean and in good condition. The Power Mixer should be turned on for a second or two, with the vacuum hose hanging free, to see that a poor vacuum is registered on the gauge (15 to 23 inches of mercury), which confirms that the vacuum line and filters are free of clogs. Now that the equipment has been shown to be in working condition, the task of mixing the plaster can begin.

Select the proper Vac-U-Mixer® mixing bucket size for the amount of Coecal® to be used. When selecting a mixing bucket, one should be chosen that will have plenty of air space at the top of the bucket after mixing the plaster. Filling a bucket too high will result in plaster being sucked into the vacuum line and fouling the plaster filter. To determine whether a mixing bucket is appropriate for the amount of plaster needed, the dry plaster capacity should be checked. The 200ml bucket holds 25-100g, the 300ml bucket holds 25-150g, the 500ml bucket holds 50-350g, the 875ml bucket holds 50-800g, and the 1200ml steel bucket holds 100-1000g of dry plaster.

To mix the plaster slurry, the liquid hardener is emptied into the mixing bucket first. The dry Coecal® is added next and mixed together with a wooden tongue depressor for about 10 seconds to make sure no dry Coecal® lumps remain. The excess plaster is wiped off the depressor into the bucket and the depressor set aside to be rinsed and reused.

Any plaster adhering to the Vac-U-Mixer® bucket's rim should be wiped off with a finger. The lid is placed onto the bucket and brought to the Power Mixer. The vacuum hose is set into the fitting on the lid until it is firmly seated. The bucket is lifted up to the bottom of the Power Mixer, and the drive pin in the drive shaft of the mixer aligned with the corresponding notch on the shaft on top of the Vac-U-Mixer® bucket. When the bucket is raised, the Power Mixer will start automatically. The vacuum gauge should be checked; within seconds the vacuum should be pulling 28 to 29 inches of mercury. If not, while the mixer is still running, thebucket can be squeezed more tightly against the lid. The hose should be checked to be firmly seated in the lid.

The bucket should be left on the Power Mixer for about 15 to 30 seconds at 28 to 29 inches of mercury. At that point, the power is switched to 'manual on' and the bucket lowered from the mixer. The vacuum hose is then removed from the lid of the Vac-U-Mixer® bucket. As a final check to ensure that a proper vacuum was achieved in the bucket, a distinct soft "pop" should be heard when removing the hose. If the equipment was in proper working order and the gauge registered the proper vacuum, yet the vacuum hose just quietly slid out, the gauge again should be checked again now that the hose is removed from the bucket. If the gauge still reads over 25 inches of mercury, then it's most likely that vacuum within the bucket was not achieved because the plaster level in the bucket was too high, leading to plaster being sucked up into the vacuum line and clogging the plaster filter. In all cases, this problem can be easily avoided by choosing a sufficiently large mixing bucket.

Let the Power Mixer run for a minute or two after removing the mixing bucket and detaching the vacuum hose. Doing so will increase the life of the Power Mixer's® vacuum pump. During mixing under a vacuum, water vapor is drawn into the vacuum pump. If the Power Mixer were to be shut off immediately, the vapor would condense inside the pump. By running the Power Mixer without a load for a minute or so, air is drawn through the pump, thus flushing the vapor out. The air flow also draws oil vapor from the oil reservoir into the pump keeping it lubricated. When the time has elapsed, the power should be turned back to 'automatic on', which will shut the power off until the next time a bucket is inserted.

The contents of the mixing bucket are emptied into a reusable, easily cleaned bowl such as a rubber FlexiBole®. The mixing bucket, lid, and tongue depressor are rinsed and cleaned under running water in a sink equipped with a plaster, or sediment, trap. A trap that works well with plasters like Coecal® is the Coe-Bilt® plaster trap. A plaster trap is essentially a large, removable bucket immediately beneath the drain on a sink which allows the solid particles of plaster to settle out before flowing into the sewer pipes and hardening there. The trap can be removed regularly so that the settled solids can be emptied.

The plaster slurry, evenly mixed and free of entrapped air, is ready to be used for casting. As it will only stay in a workable state for 7 to 9 minutes from the beginning of the mixing process, speed and efficiency through the next several steps are warranted.

The wetting agent is emptied from the mold halves, leaving a wet film in the mold. If the Coecal® were poured into the mold now, many small bubbles would result in the final cast. The wetting agent serves to assist the technician in avoiding entrapped bubbles, but it is not a guarantee that the bubbles will be automatically eliminated. To work properly as an interface, a first coat of Coecal® will have to be applied to the mold to take advantage of the wetting agent's surfactant properties. This coat will be blown into the fine morphology of the specimen's impression. The excess will then be drained out of the mold, leaving only a thin, wet film of plaster covering the fine detail in the impression.

To apply this first coat, Coecal® is drawn into a Monoject® 12cc syringe designed with a curved, tapered tip. The tip should be trimmed somewhat (about 1/4 to 1/3 removed) to enlarge the opening in order to prevent the tip from clogging. The syringe can then be used to quickly spray a rough coat into the specimen's impression. This rough coat should take no more than 2 to 5 seconds per mold half to accomplish, and need not cover all the morphology. With the air gun set to a moderate volume of air, the plaster is worked into all the fine features, teeth, and vertical surfaces of the impression. The mold halves and the air gun may have to be held at varying angles to ensure the best results. It is important that not too much air be blown in any one place, because the plaster slurry may begin to dry in that place. When all surfaces are covered with a thin layer of wet plaster, the mold half is held on an incline and the air gun used to drain out most of the excess plaster slurry/wetting agent solution. Leaving the diluted slurry in the mold will change the overall dilution of the plaster slurry when filling the mold, which may result in discolored efflorescences on the highest portions of the cast, thereby ruining the cast.

Immediately after the molds have been initially coated and blown, filling the mold halves with Coecal® can begin. Once again, Coecal® is drawn into a Monoject® syringe, with care taken to ensure that no air bubbles are drawn into the syringe. One of the mold halves is placed onto a vibrating platform, such as the Buffalo #200 Vibrator, and the vibrator is set to a high setting. With the syringe, work begins by filling any areas of fine morphology or relief, such as teeth, muscle ridges, or broken edges. The lowest features in the mold are attended to first, and work proceeds upwards. If any air bubbles are suspected to be entrapped, a small, blunt-tipped dental tool can be used to probe for and remove them. While the bubbles cannot be felt with the probe, they often lodge in predictable places - the tips of teeth and other fine features, and sharp ridges and abrupt edges, including the outer edges of worn teeth. Once all the fine morphology has been filled, each half of the mold can be slowly filled with Coecal® while remaining on the vibrator. The plaster can either be added with the syringe, if the mold is small, or poured directly from the FlexiBole® if the mold has a larger volume. When the Coecal® is added, care must be taken to begin filling the mold at its highest, most shallowly sloping point, and to let the plaster flow into all the other areas of the mold at its own rate. The vibrator, if on a high setting, should make this part of the process proceed quickly.

When each of the mold halves is roughly full, the vibrating platform is turned off and the mold halves placed on the countertop, near the edge, where they can be picked up easily. Each side of the mold is topped off with Coecal® so that each side is overfilled. Any remaining plaster can be emptied into the solid waste container. The empty syringe and plaster bowl can be tossed into the sink to be rinsed later. Carefully keeping the molds level, both halves are lifted slowly and held from the bottom.

The mold halves are then checked to ensure that they are properly aligned with each other. This process is greatly facilitated if the mold was made with tabs placed asymmetrically so that proper orientation can be ascertained even if both halves of the mold impression are completely obscured by plaster. In one, quick, smooth motion, the two mold halves are slapped shut against each other. They are then squeezed tightly against each other to ensure proper registration while forcing excess plaster to drain from the mold. The closing motion should ideally be executed similarly to the way an open hard cover book is slammed shut. Each half should pivot quickly from a level, horizontal position to a closing position in a vertical plane. If done quickly and smoothly, inertial forces will keep the plaster in position in the mold halves until they join and are squeezed together. A common mistake to be avoided is trying to get a head start on the close by tilting the mold halves first; the plaster will drain out while in this position. Proper closing of plaster-filled molds is a skill which is difficult and sloppy at first, but it is quickly learned and made more efficient with practice. It is advisable to make several dry, practice closes with each mold before casting, in order to become familiar with the technique.

From the point at which the mold is closed to the point at which the Coecal® sets, pressure must be applied evenly over the entire mold. Usually this means holding the mold with both hands, one squeezing each side of a mold; or if the mold is small, with one hand holding the mold and applying pressure in a more or less central position on the mold.

Once the mold is closed and pressure has been applied to drive out the excess plaster, the mold should be rinsed in clean water to remove wet plaster adhering to the outside of the mold. While holding the mold firmly to ensure a watertight seal, plunge the mold under running water in a sink equipped with a plaster trap. After the mold is cleaned, it will need to have pressure applied until the plaster has set. It is important to keep constant, even pressure on the mold, especially while rinsing. Lack of sufficient pressure may allow plaster to drain or water to be admitted to the mold. Excessive pressure, in addition to introducing distortion, may, if pressure is reduced before the plaster is set, create negative pressure within the mold which will draw in outside air or water.

The Coecal® will set approximately 7 to 9 minutes after the initial mixing, a time which can be determined either by using a timer or by observing the setting of the excess plaster which remains on the casting countertop. Even pressure must be applied to the mold throughout this time. This can be done in any of several ways - by hand, by setting the mold on the counter and placing weights on it, or by placing specially designed rubber mold bands around the mold. The best way to ensure good results on the finished cast is to use at least 2 rubber mold bands that properly fit the mold both lengthwise and widthwise, and to place these bands on the mold immediately after rinsing the mold. The bands will supply pressure evenly and at many points across the mold. Additionally, the mold is capable of being rotated freely to keep the Coecal® slurry homogenous within the mold until it sets. This rotation will be especially helpful if the plaster was mixed a little thin or if so much plaster spilled out upon closing the mold that a void is expected inside the cast. If properly sized bands are not available, the mold can be held shut by hand until the plaster sets. The mold is also free to be rotated with this method. A third method is to simply set the mold on the countertop and slowly replace the pressure of the hands with a flat weight. Lead SCUBA weights coveredin a rubber safety coating work well for this purpose. The 5 lb weights work well singly, or in pairs, for most moderately sized molds, while the 2 lb weights are more appropriately sized for the smaller molds.

Once the Coecal® has set, the mold can be gently set on the countertop (or if the weights have been used, left on the countertop) for an additional 10 to 20 minutes or more to allow the Coecal® to fully harden.

After the cast has hardened, it can be removed from the mold. Demolding is begin by removing the plaster jackets and any blocks from the rubber portions of the mold. While holding the mold over a soft, clean surface, the flanges of the mold are slowly opened. Working slowly from the point of least resistance, work proceeds around the circumference of the mold until the flanges of the mold are separated all the way in to the cast. Trapped between the two sides of the mold, from the edges right up to the specimen, will be a thin layer of plaster called the flash. This can be broken off and discarded as work advances around the mold.

Once again working from the point of least resistance, the rubber can be gently pulled away from the sides of the cast. At this point on most molds, one of the mold halves will spring away from the cast, allowing the cast to be directly manipulated and removed from the mold. In certain cases, as with cranial vaults, the mold will need to be peeled off the cast, being turned inside-out as it progresses. In others, such as deep, narrow molds of items like incisors, pressure may have to be exerted on the cast through the rubber in order to remove the cast from the mold. Once the cast is removed, all that remains to be done is to clean and reassemble the mold and to check the cast for quality.

All surfaces of the rubber portions of the mold, the plaster jackets, and the blocks, if any, should be cleaned of any adhering plaster. The mold label on the plaster jackets should be checked to be legible and a Glyptal® coating should remain over all exposed plaster surfaces. If not, jackets are relabeled and resealed where necessary. If the mold is wet, it is left out to dry before storing it away to prevent fungal growth inside the mold.

A moment can now be taken to look over the cast for defects. If possible, the cast should be compared with the original specimen. The flashline, if done properly, should ideally be 0.5 mm thick or less, although flashlines up to 1 mm thick or more may be acceptable for certain non-research purposes. Bubbles and beads on the surface of the cast, as well as for subsurface voids, should be looked for and noted. If the defects seem easily overcome without any substantial damage to morphology, they can be patched or repaired. Such touching-up work will be covered in chapter 5. If the defects are greater than this, the cast is discarded or put to non-research purposes.

ContentsPrefaceChap. 1Chap. 2Chap. 4Chap. 5Chap. 6Epilogue
GlossarySafetyMaterialsSuppliersBibliographyMat. Specs. .