(C) 1991-2009, Glen J. Kuban
Liquid latex rubber can be used to make a flexible mold of almost any firm, non-porous subject. When properly made, a latex mold will record even microscopic details, and yield a sturdy mold from which many casts can be made. Liquid latex is also less expensive than most other rubber molding materials such as silicone and urethane rubber. A latex mold is also very elastic--able to be turned inside out if needed. The procedures for making a latex mold are not difficult, but can take several hours or more. One should also work outdoors or with good ventilation, since liquid latex emits ammonia fumes. Latex molds also shrink with age (normally about 3 to 5%), and because it is a natural rubber (derived from rubber trees) they eventually decay and deteriorate. Therefore they should be cleaned after every use, and stored in a clean, dry, dark place to maximize shelf life. When properly stored and cared for, a latex mold can remain serviceable for several years or more, and produce scores or even hundreds of casts. Rubbing the molds periodically (every few months or so) with castor oil will somewhat rejuvenate them and extend their life.
These instructions assume the subject to be molded is a fairly simple, one sided object such as a dinosaur footprint. More complicated, multiple-sided subjects such as skulls require additional procedures beyond the scope of this summary. The basic procedures are:
1. Clean and prepare the specimen. Remove any loose dirt, moisture, or oil. Plug any deep holes with clay. 2. If the surface is coarse, apply a thin layer of release agent such as paste wax (most specimens do not need it). 3. Brush on several layers of latex, allowing latex to largely dry between layers. 4. After several layers are built up, apply a reinforcing cloth for added strength. 5. Add several more layers of latex. 6. After mold is completely dry, peel up mold from the specimen; 7. If a mother mold is needed, replace mold over original specimen, and apply rigid jacket material. 8. Remove mold and jacket from specimen.
Supplies Notes / Availability
Original specimen May be almost any rigid object. Liquid Latex Hobby & craft stores or commercial suppliers Paint brushes 1/4" - 1/2" fine-hair flat brush recommended Newspapers & paper towels To cover work area, cleanup Cups or jars To hold small amounts of latex during mold making. Water For thinning latex if needed (distilled water is best) Stirring rods Popsicle sticks work well
Optional supplies
Release agent (if needed) Paste wax or thinned soap Clean rag For buffing excess release agent Acrylic paint Add a couple drops to color the liquid latex if desired. Open weave cotton cloth To reinforce mold. May use gauze for small specimens. Scissors To cut cloth or trim mold Plaster or urethane foam For rigid jacket or backing material (help molds retain its shape) Permanent magic marker To label finished mold
1. Prepare work area. Cover table or floor with newspapers or plastic. You may want to also wear an apron or old clothes. Latex is virtually impossible to remove from clothing.
2. Prepare specimen if needed. Make sure specimen is clean and dry. Strengthen any weak, brittle, or flaky areas with glue or other consolidating compounds. Small cracks, crevasses, or holes may be filled with clay.
3. Apply release agent if needed. Most specimens will not require a release agent, unless they are especially porous or friable. If in doubt do a small test patch in an unimportant part of the specimen. Preferred release agents include paste wax or silicone spray. Do not use Vaseline, Pam, or other oil based compounds, since latex is soluble in petroleum products. Again, when in doubt, test the release agent on an unimportant area of the specimen. Using a brush or clean cloth, work the material into all pores, cracks, and undercuts. Buff out any excess release agent. If using soapy water, allow it to dry before applying latex (steps 5 and 6).
4. Prepare the latex. Pour some latex into work jar or cup. If latex is thicker than motor oil, gently mix in a small amount of water. Distilled water works best. The latex should be free-flowing for the first 2 coats; for later coats a thicker consistency is OK. Cover work jar if latex is not all used, or skin will soon form.
5. Apply the first coat of latex, using a fine-haired brush (a 1/4"-1/2" inch flat brush is recommended). Spread the latex gently and evenly over the entire specimen, being sure to cover any undercuts or hard-to- reach spots. Use smooth, light brush strokes to avoid creating air bubbles. Do not repeatedly go over the same areas (otherwise you may lift up the first layer). As this first layer dries, it will become almost transparent. Each layer typically takes several minutes or more to dry sufficiently for the next to be applied, but drying time varies depending on humidity, temperature, and amount of air circulation.
6. Apply additional coats of latex after the first coat is completely dry. Be especially careful when adding the second coat to avoid damaging or lifting up the thin first coat. Allow each layer to dry or almost dry before applying the next, but do not wait more than an hour or so after each layer is dry before applying the next coat, or it may cause separation between the layers You can apply latex in a little thicker after the first two layers, although the thicker the layer, the longer it will take to dry. You can mix a couple drops of acrylic paint into the latex to tint it (stir well but do not froth it). The color will help distinguish new and dry areas as more coats are applied. Its best not to add color until the third or forth layer, otherwise the specimen may be stained.
Application of low heat or blowing air (or both, using a hair drier) will speed drying, but may create undesirable air pockets (if time permits, let it dry on its own). It’s best to let the latex air dry. Build up at least 4 or 5 layers before adding reinforcing material (see below).
7. Reinforcement layer. When at least 4 or 5 layers have been applied, and the coat is starting to dry (but still damp), apply open weave cotton cloth by laying it on the latex, and brushing on more latex atop the cloth. This will strengthen the mold and help it keep its shape, but will also make it less elastic (if you need to turn a deep mold inside out, you may want to skin the reinforcement. However, it is recommended on fairly simple "one- sided" specimens. If the specimen is fairly small with and with minimal topographic relief one might use only one or two pieces of cloth (it helps to cut slices into it to facilitate conforming to contours of the specimen). If the piece is large, one can use more pieces and heavier cloth (burlap works well for very large specimens). When applying the cloth and brushing on more latex, make sure the cloth conforms well to the contour of the specimen.
8. Final layers. Add several more coats of latex, letting the latex mostly dry between layers. The entire mold should contain at least 8 coats, and preferably 10 to 15, depending on whether a reinforcing material is used, and how much flexibility is desired. The finished mold should be at least 1/16 inch thick, with no transparent or translucent spots.
9. Dust a little talcum or anti-fungal foot powder around the edges of the mold and brush along the latex before peeling it up. This will prevent the thin edges from curling and sticking together. Anti-fungal power (such as a foot powder) will also inhibit microbial decay of the mold.
10. Gently peel the mold from the specimen after the last latex layer is completely dry. Lift edges first, then work toward the inside. If parts of the mold are stuck in crevasses, tug with a gentle vibrating motion. Use scissors to trim any ragged or thin mold edges if desired.
11. If a rigid backing (also called a jacket or "mother mold") is needed, first replace the mold back over the specimen. A rigid backing is normally required on complex or deeply impressed specimens to ensure that they hold their shape during storage and cast making. Do not apply any backing unless the mold is first peeled up and then replaced back over the specimen. Otherwise the backing/jacket may be very difficult to lift up. Common choices for backing materials include plaster, resin, or expandable urethane foam. Expandable foams are mixed in two parts like resins, but expand rapidly upon mixing (thus, one must contain them as they expand. On a track, one normally places wax paper over the expanding foam, then use a flat board to prevent the foam from oozing out of the track. In small quantities urethane foams can be purchased in hobby stores under the brand name "Mountains in Minutes," or from some craft stores as artificial flower pot-filler. Larger quantities can be purchased from industrial suppliers and taxidermy supply houses. Caution: when using expandable foams or other urethane-based materials, always work outdoors or with good ventilation. The fumes from most urethane compounds are dangerous.
12. Remove the finished mold and backing. Check for any thin areas that may have stuck together. Peel them apart and dust with talcum powder; otherwise they may get stuck together permanently.
13. If using a "prevulcanized" latex, a cast may be made within hours after mold removal. Otherwise, wait at least 2 days, or boil the mold for several minutes in plain water, to vulcanize it before making a cast.
14. Clean up work area, specimen, and brushes. If you rinse your brush in soap and water immediately after use, it may be salvageable; however, it is difficult to remove all the latex. Soaking brushes in kerosene or Coleman fuel for a few hours will greatly soften and expand the latex, and allow it to be combed out. However, It is usually easier to inexpensive, disposable brushes, and just discard them after use.
14. Make a cast from the mold if desired. Casts may be made of Plaster of Paris, gypsum cements such as Hydrocal or Hydrostone (from US Gypsum Corp.), resins, or other materials. (See separate instructions on casting).
Notes on Storage:
Always clean and dry latex molds after use. Store molds in a dark, dry location, at or below room temperature, but not below freezing. Molds should also be stored between a supporting jacket and a cast (creating a mold sandwich, so that they do not warp or loose their shape. Dusting molds anti-fungal powder will retard mildew and help prevent thin edges from sticking together. Do not store latex molds in paper bags; the acids in the paper can accelerate deterioration. Do not expose the latex to oils or petroleum products of any kind, which can weaken them and accelerate decay.
Cementex
480 Canal St.
New York, NY 10013
http://www.cementex.com
Phone 800-782-9056
Chicago Latex Products
1030 Morse Ave.
Schaumburg, IL 60193
http://user.mc.net/~spartan/cl.htm
Phone 708-893-2880
Perma-Flex Mold Co., Inc. (Also carries silicone rubber)
1919 E. Livingston
Columbus, OH 43209
http://www.permaflex.com
800-736-6653
Most craft and hobby stores sell latex in pint containers for about $8.00 - $10.00. It is sometimes sold as rug backing. Make sure the label indicates "Pure liquid latex."