Making Silicone Rubber Molds

(C) 1991-2015, Glen J. Kuban


Silicone rubber is an ideal material for making molds of fossils and other rigid objects. Like liquid latex, it yields a light, flexible, high-fidelity mold, but has the added advantages of longer life and resistance to chemicals and decomposition. It is the recommended material for making long-lasting molds of important specimens. A silicone mold also can be made in less time than an latex mold, if "fast" catalysts are used. Among silicone's few disadvantages is that it is considerably more expensive than latex, and not quite as elastic or tear resistant.

The most common silicone compounds used for mold making are RTV or "Room Temperature Vulcanizing" silicones which are mixed in two parts (a base and a catalyst) to induce curing. The silicone mixture is poured or spread over a prepared specimen, with gauze or other reinforcing cloth added between pours for increase strength and tear resistance if desired. After curing and removal, the mold may be replaced and covered with rigid jacket or mother mold (often composed of plaster, resin, or urethane foam) to preserve the original mold shape during storage and casting. Normal curing time for most silicones is between 18 and 24 hours, but cure times may be greatly reduced by using fast-acting catalysts. When making molds in a laboratory, vacuum deairing may be performed to remove trapped air bubbles; however, when working in the field or without deairing equipment, alternative techniques (discussed below) may be used to minimize air bubbles.

This summary reviews basic procedures for using 2-part RTV silicones without deairing equipment, including some tips from personal experience. It assumes that the specimens to be molded are relatively simple "one-sided" specimens such as dinosaur tracks or fossils on matrix. To make molds of more complicated, multi-sided fossils, see Smith and Latimer (1989).

I recommend reading the summary all the way through before making a silicone mold for the first time, to get an overall understanding of the procedures and techniques. I make no guarantees for any of the products or methods described herein, but have used them successfully in the past. Note that some silicones may slightly discolor some rock types. The reader is encouraged to test and experiment with unimportant specimens before using them on valuable subjects. The procedures may seem complicated at first, but become relatively simple with practice.

Safety Note: RTV silicone rubber compounds are relatively safe when properly used; however, the curing agents or catalysts may be toxic if ingested and area irritants to eyes and bare skin. Always wear rubber gloves and work with plenty of ventillation when working with silicone and urethane compounds.


Among the manufacturers of high quality RTV silicone rubber compounds in the U.s. are GT Pruducts, Silicones, Inc, Dow Corning, Inc., and General Electric. Each makes a variety of silicones and catalysts with various viscosities, colors, and other features. Smooth-On Corp. distrubutes a wide line of silcones made by other companies but forumalted and marketed under their own name.

There are two main classes of RTV silicones: 1. Tin catalyzed or "condensation cure" silicones and 2. Platinum catalyzed or "addition cure" silicones. Each has its benefits and drawbacks. Tin-cured silicones are generally less expensive and easier to use. They are typically of low to medium viscosity (easily poured) and are not inhibited by many materials. In contrast, platinum cure silicones are inhibited by many naturally occuring compounds, especially sulfur, tin, and amines. This makes them unsuitable for many natural objects such as fossils in matrix, unless the subject is sealed well first. However, platinum- cure silicones have the greatest chemical, microbial, and temperature resistance, and generally remain flexible for years. In contrast, tin cured silicones tend to become somewhat brittle after a few to several years, and begin to split or tear. For these reasons silicones in the tin group are often used for field work and low-volume plaster casting; those in the platinum group for important lab work and resin or epoxy casting (especially in high volumes). Also available are one-part, self-curing silicones for sealing and caulking, which may be purchased at any department or hardware store. These have some special applications in casting, but in general are not recommended (discussed further below).

For mold making in the field, or quick and inexpensive mold making in the lab, I recommend a low to medium viscosity, tin-catalyzed silicone, such as GI-1000 by Silicones, Inc, or Most silicones are available in 1 lb. cans, and 10 lb. (or 1 gallon) cans, which include a "standard" catalyst for typical cure times between 16 and 24 hours. Fast catalysts (discussed below) may be purchased separately. Catalysts are mixed with the base in a prescribed ration depending on the silicone variety. Some use a 50:50 ratio of base to catalyst; others a 10:1 or 100:1 ratio). As of this writing most silicones cost from around $100/ gal, depending on the brand and particular silicone, and the quantity purchased. Silicones Inc. and GT Products silicones are generally cheaper than GE or Dow, but of similarly good quality. Note that RTV silicones and their catalysts have a limited recommended shelf life, ranging from 6 months to 1 year according to the manufacturers, so it is best to purchase only what you can use within a few months. However, I have found that many silicones may be used successfully up to 5 years from the date of purchase if properly stored in air-tight containers in a cool, dry location. However, catalysts seem to loose their effectiveness more quickly even if stored properly, and are best used within a year. One can buy catalysts separately if needed to replace expired ones.

Fast catalysts are available for each which can reduce cure times significantly--in some cases to under 1 hour. One may also combine slow and fast catalysts (partial doses of each) to achieve intermediate cure times and assure even curing, although the practice is not endorsed by the manufacgures. With most silicones, there is some latitude allowed in the catalyst dose (adding more than the recommended amount will speed cure times, as will using a fast catalyst). However, there are limits to the amount that may be added (discussed further below), and adding more than the recommended amount of caralyst, or using a fast catalyst, will shorten the life of the mold, making it more prone to tearing or becoming brittle over time. A catalyst-rich mold may last a year or so; one with less catalyst may last for years.

Thinners or "diluants" are available for thinning or decreasing the viscosity of some RTV silicones, as are thickening or "thixotropic" agents for increasing viscosity. However, they are expensive and do not seem very effective (one must add a large amount of diluant to achieve a small effect on viscosity, and thorough mixing is difficult and time consuming). Diluents also weaken the cured silicone. I recommend buying silicone in the desired viscosity rather than using diluants or thickeners.

Thick-consistency silicones are also available as one-part sealing/caulking silicones mentioned earlier, which come in tubes and are available anywhere hardware or building supplies are sold. They are cheaper than 2-part RTV silicones, but have several disadvantages: 1. They adhere strongly to most surfaces, requiring liberal application of a release agent, 2. They are extremely thick (making application without trapping air bubbles difficult), and 3. Most cure by releasing acetic acid, which is an eye irritant and may damage some substrates. 4. They tend to cure slowly (24 hours or more) and unevenly. The cure time can be greatly reduced (to under 1 hour) by mixing in a small amount of distilled water (approx. 1 teaspoon per pound). The water must be mixed quickly and thoroughly to prevent uneven curing. Warning: Never mix water with two-part RTV silicones- -it will have the opposite effect and inhibit curing. The only time I would recommend one-part silicones for mold making is when working in a wet or moist area in the field For more information on using 1 part silicones for mold making, see Maceo and Riskind (1989).

Specimen Preparation

A specimen to be molded should be well cleaned and dry. If the specimen is friable, weak, or porous, one may need to apply a consolidant or sealant, and possibly plug deep holes or crevices. Common consolidants include melted polyethelene glycol, and polyvinyl acetate (or Duco cement) dissolved in acetone, toluene or alcohol (requiring good ventilation). Deep holes or cracks can be plugged with cotton covered with a consolidant, or with a sulfur-free oil-based modeling clay, available from arts and crafts supply houses or direct from the manufacturer. For sealing a porous surface clear acrylic or polyurethane spray may suffice.

If a specimen is especially porous or rough, or has many crevasses and undercuts, or is prone to spalling, a thin layer of a release agent (such as paste wax or thinned Vaseline) may also be recommended to facilitate mold removal. However, on most solid, sturdy specimens, no release agent is needed with most silicones. Always apply any sealant (and wait for it to dry) before applying any release agent. The release agent should be applied over the entire specimen (be sure to get it into all nooks and crannies), but only thinly, so as not to compromise surface detail. If using a brush to apply the agent, buff it afterward to remove any excess and erase any brush stokes. Silicone rubber records even microscopic detail, so you will in effect be recording the surface of the release agent rather than the specimen. Besides petroleum jelly (which may be thinned with acetone or other petroleum solvent for easier application), other suitable release agents for silicone include paste waxes and commercial release agents based on wax, liquid silicone, Teflon, or synthetic oils (unlike latex, most silicone compounds are not weakened by petroleum compounds). However, some release agents may slightly darken or discolor a specimen. This may be only temporary and may disappear or lessen as the compound evaporates over time. Again, it is best to test all substances on unimportant specimens.

Building a Retaining Wall

Unless a specimen has natural boundaries to contain the silicone during pouring, one will need to build a retaining wall around the specimen. This can be done with almost any inert material (wooden planks, plastic stripping, cardboard sealed with clear tape, etc.) Tape may be used to connect sections of the wall, and sulfur-free clay or latex rubber often works well for sealing any small openings between sections or along the base. Poured silicone rubber will find its way through even the tinniest openings, so one cannot be too careful with this step.

Mixing the Silicone

Because of settling during storage, always stir the silicone in the original container before decanting into a mixing cup. Stir steadily with a circular motion rather beating it or using a up-and-down motion, to avoid trapping air bubbles, scraping the bottom of the can to loosen any settled material. Then let the silicone rest for a few minutes or more to allow air bubbles to rise to the surface (this small resting time will not cause any re-settling of the silicone itself). Tip: Tongue depressors or craft sticks make good, cheap stirrers for small batches; paint stirrers make good stirrers for larger batches.

Next, decant the desired amount of silicone for into a separate mixing container, such as a plastic cup or wax-free paper cup. Select mixing containers with relatively straight bottoms and sides, and little or no inner lip. In order to save time in the field, the first two steps can be done ahead of time: the silicone base can be decanted into straight-sided plastic jars (with lid threads only on the outside), that can be used for storage and mixing (after pouring, any remaining silicone in them will cure and can be pulled out, allowing them to be reused again and again).

To estimate the amount of silicone required, the following guidelines may be helpful. For a relatively flat fossil with shallow relief, one may wish to pour enough silicone to fill up the entire volume of the specimen, leaving a flat mold surface (the future backside of the mold). This will avoid having to use any type of backing or rigid supporting material after the silicone has cured. For larger specimens or ones s with severe contours, it is usually impractical to make this kind of mold. Instead one may merely to cover the surface of the fossil (along with any holes or crevices) to about 3/16 inch or so, and then use other materials (explained below) to provide rigid support and fill up the remaining volume (also this saves silicone and money. However, try to cover the entire specimen to at least 3/16 inch in all areas; if any thinner the mold may tear. It is also wise to lay the silicone a little thicker in narrow or complicated sections, and along the edges, where extra strength is required to avoid tearing. As explained below, gauze can also be used to reinforce the mold.

To cover a specimen evenly without having the silicone pool excessively in deep places, it is often advisable to pour or apply the silicone in two or more batches (discussed further below). If a second batch is applied, it should be applied while the first batch is partly cured but still tacky. Once a layer of silicone is cured completely, it is difficult to make a good bond with a subsequent layer. Using multiple applications also allows gauze or other reinforcing materials to be embedded between layers, making a much stronger mold.

Adding the Catalyst

After decanting into a mixing container, mix in the catalyst. If you are not in a hurry, it is best to use the recommended dose of the standard catalyst. However, one need not be extremely precise in measuring the amount of catalyst (with practice an "eyeballed" measurement is normally sufficient). One can be off by 10 or 20 percent without much effect in the cure or final product (a little more will speed the cure, a little less slow the cure). Cure time is also somewhat affected by temperature and humidity (heating accelerates the cure). Despite the latitude in the amount of catalyst used, there are minimum and maximum limits (see manufactures specs). An excessive amount of catalyst may not allow enough work time and can result in a brittle mold; too little may cause incomplete or uneven curing.

One has at least several minutes of available "work time" time with most standard catalysts before the silicone starts to cure or "set up." Stir well for at least two or three minutes, and scrape all parts of the container to achieve a thorough mix. However, avoid overly vigorous motions that can introduce air bubbles. After mixing, one may let the silicone rest a few minutes to allow air bubbles rise to the top (which should be broken before pouring), or one can use a deairing vacuum chamber if available (discussed below). After mixing, it is best to decant the silicone into yet another container, to avoid using any of the poorly mixed silicone that often exists at the bottom and sides of the container.

Using special fast catalysts the cure times can be reduced substantially, sometimes to an hour or less. This is often handy when working in the field, where weather conditions or schedules may limit the time available. According to manufacturers, fast catalysts may be used in place of the normal catalyst; however, I have found that they are much more effective when used in combination with the slow catalyst (using a partial dose of both). If used alone and in full dose, the fast catalysts tend to be difficult to mix thoroughly in the shortened work time available, and may result in uneven curing. I have found by experimentation that one can assure both an even cure and a fast cure by first mixing in about a half dose of the recommended standard slow catalyst, and then about a half dose of fast catalyst (see Appendix B for specific ratios and cure times). That way, even if the fast catalyst is not completely mixed, all parts of the mold will eventually cure. The more catalyst that is used, the faster the cure time and the shorter the work time (the time before the mixture starts to cure). Never use more than a maximum recommended dose of the fast catalyst (or more than one half dose if used with the slow catalyst), otherwise the silicone will begin setting up before you can stir and pour it properly, and mold life will be shortended significantly. After the fast catalyst has been added, the silicone will often start setting up within minutes. Begin stirring immediately after adding the fast catalyst, and be prepared to start pouring as soon as you stop stirring. Caution: When using the slow and fast catalyst together, always add the slow catalyst first, and mix well before adding the fast catalyst. Do not reverse this order or mix the catalysts themselves together before adding them to the silicone--otherwise you will defeat the purpose of the technique and will not give enough stir time for a thorough mix.


If a vacuum chamber is available, use it to remove trapped air from the mixture before pouring. When subject to a vacuum, the silicone mixture should well up as air pockets rise and burst. As soon as the material settles down, proceed to the pouring/application procedures described below. If a fast catalyst was used, you must work quickly to avoid having the silicone cure before it is applied.

Pouring or Applying the Silicone

One of the main concerns when applying silicone to a subject is to avoid trapping air bubbles on the surface. Besides choosing a low viscosity (free-flowing) type of silicone, there are several techniques that are helpful in this regard. One is to apply a thin initial coat of silicone with a fine paint brush, gently spreading it into all cavities and undercuts (after which more silicone may be poured). Another method to reduce air bubbles (which may be used in combination with the first method) is to hold the silicone container high above the specimen and allow it to flow down slowly, in a very thin stream. This tends to break air bubbles on the way down. Yet another technique (which again may be used in combination with the others) is to temporarily incline the specimen at an angle, and pour the silicone onto the higher end, allowing it to flow down over the rest of the specimen. When the silicone reaches the lower end, then lay the specimen flat and/or tilt or rotate it in other directions as needed to achieve an even coverage. Yet another technique (sometimes used mostly by professional labs) is to use a small compressed air gun to direct small amounts of silicone across the mold surface and into any crevasses or undercuts. Using any of these methods, you may need to manually push the silicone around a bit to encourage even coverage (using a small tool such as a craft stick), and/or repeatedly pull it from the deeper to the shallower sections (where it tends to pool). As the silicone cures it will begin to "stay put."

If you find there is not enough silicone mixed to cover the fossil to a good depth, it is better to spread it in a thin layer over the entire fossil, and then apply a second batch over it, rather than to cover only part of the fossil and then fill in the second batch (the latter method tends to leave small seams between the pours). To reinforce the mold (and help avoid tearing), one may embed gauze or other open- weave cloth between layers, as described below.

Reinforcing the Mold

After the silicone has started to cure, but while it is still tacky, one may gently apply strips of gauze, cheese cloth, or open-weave cotton cloth to increase the strength of the mold, especially if two layers are being laid down. Be sure not to push the gauze through to the specimen surface. After the gauze is applied, apply another layer of silicone to thoroughly cover the gauze layer. Typically the finished mold should be at least 1/8 inch thick even in the thinnest sections. Thinner molds are sometimes useful for specimens that may need to be turned inside out to remove. One may also wish to only reinforce the edges of the mold (especially any thin edges), which are more prone to tearing than the main body of the mold. When selecting gauze, be sure to get the non-elastic type (the "stretchy" kind commonly sold today tends to bunch up).


Normally the surface of a silicone mold (opposite the specimen) will be smooth and shinny, making it difficult to write on. However, one can create a textured, markable patch on the silicone as follows. After the silicone has begun to cure but before it is firm, gently smooth onto it a piece of paper or index card. If should float on top of the curing silicone. If it begins to sink it, gently remove it and wait until the silicone has cured a bit longer. After the mold is fully cured, one can peel up the paper, and the dull- textured area underneath will be suitable for making with a good permanent magic marker (such s Sharpie). It is wise to record identification information for the specimen, as well as the molding date, and the precise silicone product used. The latter information will help if repairs have to be made latter, since silicones adhere best to each other when they are of the same manufacturer and type.

Removing the Mold

After the prescribed cure time has elapsed and the outer surface of the mold feels firm and dry, the mold is usually ready for removal or demolding. However, if the mold has any deep areas, or time is not of the essence, it is best to wait a bit longer to ensure that all areas of the mold are fully cured. When removing the mold, gently peel up all the edges first, and then the middle section. If any portion seems too soft, immediately stop pulling and replace any lifted sections, allowing more time to cure further.

Adding a Rigid Backing

Regardless of whether any gauze reinforcement is applied, most molds will need some type of rigid supporting structure (sometimes called a "mother mold", backing, or jacket) to ensure that the original mold keeps its shape during storage and casting. Small, uncomplicated molds, or ones that can be poured so that the backside is level, may not need such support. However, in other cases a mother mold is recommended. The mother mold may be made of expandable foam, casting plaster, urethane plastic, or fiberglass resin. Before pouring the mother mold material, make sure the original mold is fully cured, lifted off the subject, and then replaced back onto it (otherwise it will be difficult to lift the original mold off the subject).

The cheapest and fastest way to create a mother mold is with a Plaster-of-Paris or gypsum cement jacket. The plaster may be simply poured over the mold (with a retaining walls created to contain it if necessary), or applied in "plaster bandages" (strips of burlap or open-weave cloth soaked in plaster slip). However, plaster jackets are heavy, prone to breakage, and has no flexibility (which may be needed on some molds).

An good alternative is expandable urethane foam, which comes in two parts (mixed in equal amounts) and expands when mixed to fill any volume. It is available in several different densities (versions of it are often used for boat floatation, insulation, and packaging). Among the advantages of urethane foam is that is very quick, and produces a light-weight, sturdy, and slightly flexible support. This is especially useful when working with large specimens and/or when traveling (where weight is a major consideration). Some disadvantages are that it is relatively expensive, and produces harmful fumes when expanding (it should be used only outdoors or with very good ventilation). It also sticks strongly to any substance not well- lubricated --if it gets on your skin it has to wear off. However, when properly used it is an ideal filler and backing material. In small quantities (pint and quart cans) it can be purchased at hobby stores under the brand name "Mountains in Minutes." For larger quantities see a commercial casting supply company or a taxidermy supplier. One should experiment to understand how much foam is required to fill a given volume (when fresh it expands 5 to 20 times the liquid volume, depending on the specific product).

The procedure for filling the volume of a deep mold with expandable foam is as follows:

1. Have supplies handy (urethane, release agent, wax paper, large flat board, paper towels).
2. Pull up and then replace the mold back over the specimen.
3. Apply a layer of release agent (paste wax or Vaseline) to the mold.
4.. With good ventilation, mix equal volumes of the two foam liquids ("A" & "B") in disposable paper or plastic cups. Mix vigorously (air bubbles are fine with the foam).
5. As soon as the foam starts to expand (or after 2 minutes of stirring, whichever comes first), quickly pour the mixture onto the mold (pour more of the foam in the deeper areas, less in the shallower areas).
6. As the foam begins to expand, place large sheets of wax paper over the foamed mold, and then press down with the flat board to contain the expanding foam. The force of the rising foam can be surprisingly strong. You may have to sit on the board or pile on heavy weights to keep the foam from lifting the board and oozing out. (Experiment first to determine the amount of foam needed).
7. The foam should calm down after about 5 minutes, but continue to react somewhat for another few minutes (keep the weighted-down board on it during this time). After about 15-20 minutes the foam should be relatively firm and rigid.
8 Remove the foam and mold. If you must trim the foam, avoid inhaling any of the foam particles.

Caution: Never use expandable foam directly on a specimen even with a release agent. It can partially eat through release agents and stick to specimens. Avoid getting the mixed urethane foam on clothing and skin--it will adhere with a vengeance. Using disposable rubber gloves when mixing expandable foam is a good idea.

Cleaning the Mold and Specimen after mold-making

After removing the mold, be sure to clean it and the specimen before storing either. If you used Vaseline or other oil-based release agents, or silicone without a release agent, the residue or discoloration can often be removed or reduced by washing the specimen with kerosene or other petroleum solvents. Avoid getting solvents on your skin, and always use plenty of ventilation. Be sure to test an unimportant specimen first to be sure the solvent does not dissolve the matrix or otherwise worsen the situation.

Making Finished Casts from a Silicone Mold

Rigid casts may be made from silicone molds using casting plasters (such as Plaster-of-Paris or Hydrocal), plastics, resins, epoxies, temperature metal casting (check manufacturer’s product literature for temperature ranges). Normally a release agent is not needed for plaster casting, but for urethane and resin casts, or complicated molds with severe undercuts or many crevasses, a release agent such as thinned Vaseline and or barrier coat (such as a varnish or paint applied to the mold before casting) is advisable. Depending on the nature of the mold, one may need to build a frame around it to retain any overflow casting compound. Mix and pour the casting material according to manufacturer's instructions. After the cast has hardened completely, slowly peel the mold from the cast. Work around all the edges before pulling up the middle sections. If the cast is deep, the remaining volume may be filled with expandable foam if desired, the same way it may be used to fill the volume of deep molds. The finished cast may then be painted (acrylic paints are commonly used), or otherwise prepared for display.

Cleaning and storing the Mold

After a cast is made, be sure to thoroughly clean the mold (removing any casting residue or release agent) before storing. Store silicone molds in a clean, dry location, embedded between both the rigid mother mold and a "retainer" cast. This creates a sandwich effect that prevents the mold from distorting or warping over time.

Summary of Silicone Mold-making Procedures

1.  Clean and prepare specimen (apply consolidant, sealant, or release agent if necessary). 
2. Build retaining wall around specimen.
3. Stir silicone base to loosen any settled material.
4. Decant desired amount of silicone into mixing container.
5. Mix in appropriate amount of catalyst.
6. Apply to specimen.
7. If reinforced mold is desired, gently press in gauze after pouring fist layer.
8. When cured, gently remove mold from specimen.
9. If needed, create rigid backing or jacket over mold with plastic, urethane foam, or plaster.
10. Remove backing material and mold. Store mold inside rigid backing.


Chaney, Dan S., 1989, Mold Making with Room Temperature Vulcanizing Silicone Robber, in Feldmann, Rodney M., Chapman, Ralph E., and Joseph T. Hannibal, Paleotechniques, The Paleontological Society Special Publication No. 4, Department of Geological Sciences, University of Tenn., Knoxville, TN., pp. 275-331.

Dow Corning Corporation, 1987, Silicone Moldmaking Materials from Dow Corning, Midland MI.

Maceo, P., and David Riskind, 1989, Field and Laboratory Moldmaking and Casting of Dinosaur Tracks, in: Gillette, D., and Martin Lockley, Dinosaur Tracks and Traces, Cambridge University Press, NY, pp. 419-420.

Smith, J., and Bruce Latimer, 1989, Making a Multiple Piece Mold, in Feldmann et al, Paleotechniques (cited above).

APPENDIX A: Silicone Manufacturers and Distributors

GT Products    (will sell direct) 
Grapevine, TX 76051
(800) 221-0866

SILICONES, INC.  (Will sell direct)        
High Point, NC   27261

SMOOTH-On Corp (Sells through distributors)
Macungie, PA

DOW CORNING CORP. (sells through distributors)
Midland, MI  48686-0994
GENERAL ELECTRIC Co, Silicone Products Div. (sells thru distributors)
Waterford, New York  12188

RHONE-POULENC, INC., Silicones Division
Cranbury, NJ  

POLYTECH, INC.  (Will sell direct)       
P.O. Box 384
Lebenon, PA, 08833
(Carries some silicones but specializes in urethanes)

NETHERLAND RUBBER                     
2931 Exxon Ave.
Cincinnati, OH  45241
(Carries Dow Corning Silicones)

PHILPOTT RUBBER   (Carries GE Silicones)                                 
Cleveland, OH  44115

F.B. WRIGHT CO.                     
Ceveland, OH
(Carries GE Silicones)
Freeman Mfg, 
Cleveland, OH