Molding and Casting

 These are usually made in either silicone or polyurethane rubber. The cost of these materials and the difficulty in removing solid blocks of rubber from specimens with complex shapes or deep undercuts makes this method most appropriate for small specimens and/or those without a complex shape.

This technique may be necessary for large and complex shapes that require more flexibility in a mold to allow the mold to be removed safely (demolding). Thin-walled molds are usually made in silicone or latex. Silicone is used where replication of very fine detail is important. However, it is easier to tear, making it less practical for extremely large molds. Latex in used where detail is not as important. It is difficult to tear and may even break the bone before tearing.

Before molding, specimens are generally treated with a dilute adhesive called a consolidant. The consolidant can be used to both impregnate the specimen to impart internal strength (sometimes referred to in preparation as a hardener) as well as coating the surface with a thin but complete layer. This coating helps maintain the surface condition of the specimen and also acts as a release, making it easier to separate the molding material from the specimen during demolding. Two coats of 2-3% Butvar B-76 in acetone often works well for this step. For information on other consolidants see the Adhesives and Consolidants page.

One exception to this step is teeth, which are not usually consolidated; the consolidant layer can obscure dental wear patterns and other minute details. 

Fossils contain many holes of various sizes. Some of these are naturally-occurring morphological features of potential importance for research (e.g. foramina). Others are the result of weathering or old breaks. In some places, the specimen may be heavily undercut, presenting a challenge for molding. Filling voids in a specimen is a time-consuming, but critically important step and one that if not done properly can result in damage to the specimen as well as production of a less useful cast that omits important information.

When designing the mold it is important to determine whether these areas contain significant information, and so should not be covered. There will also be areas where liquid rubber can seep in but, once hardened, will cause damage when pulled out and therefore must be filled. A general rule of thumb is that if the hole is bigger on the inside than on the outside, it may need to be covered, filled or the mold must be designed around it. 

The goal of the filler is to preserve the fossil during the molding process—it should not cover any more of the bone surface than absolutely necessary to preserve the fossil against breakage. Remember that the resulting cast may be all that a researcher is able to see of this specimen.

The filler is usually scored or otherwise marked to distinguish filling from fossil. Care must be taken to remove any filling material that is not needed. A thin layer of consolidant is applied over the filling.  It may be necessary to use a combination of filler materials and techniques on a single specimen.

Choice of filling material is important: the goal is to remove the fill completely once the mold is completed. Good fillers must be easily workable, easily removable, and safe for both the specimen and the preparator. Commonly used fillers include:

• Carbowax
• Cyclododecane
• Microcrystalline wax
• Modeling Clay
• Paper/tissue
• Adhesive film

Each of these fillers has its own particular properties and not all are suitable in all circumstances.

A one-piece poured mold is generally the simplest to make. The one-piece poured technique is useful for small or partial specimens, such as a single tooth, or a fragment of jaw.  This type of mold is good when the specimen is viewed from one side, or is relatively conical. Because any object that is molded must also be removed from the mold, this technique may not be suitable for all small specimens; even with a single tooth it may be necessary to make a more complex mold if it will endanger the specimen to pull it out of a one piece mold.

For complex or larger specimens, more complex molds are needed. It is necessary to divide the specimen into two or more parts, making each part separately.  The goal is to separate the molds so that there is the least possible amount of stress on any part of the specimen. 

For the largest and/or most complex specimens single or even multi-piece poured molds are not appropriate and it may be best to make a layered mold.  These molds can be made in either silicone rubber or in latex rubber. Initially thin layers of molding material are applied in to the specimen in succession to capture surface detail, followed by thicker layers to partially fill in undercuts and provide strength. A layer of gauze or some other fabric is often added for additional strength and flexibility. Layered molds are usually multi-part and are thin-walled; this allows for the flexibility necessary to make demolding easier and less dangerous to the specimen. 

Once the mold has set, a jacket or “mother mold” may need to be constructed to help support the soft rubber of the mold. These jackets are usually made of plaster, although large jackets are sometimes made of fiberglass. The process of jacketing one-part molds with straight walls is relatively simple, but more complex, multi-part molds require multi-part jackets. Jackets help in the alignment of multi-part molds, ensuring a tight fit of all of the parts and a thin seam line.

Block making

Molds made for complex shapes may have deep undercuts or crevasses, even after the rubber has been applied.  These complicated molds may require making blocks to fill in undercuts that might cause the jacket to be difficult to remove.  A block, usually made of plaster, is used to fill in undercuts with small removable pieces which make the surface less irregular.

Demolding the specimen can be scary for the preparator and dangerous for the specimen as this process carries the greatest potential for breakage and damage.   Great care must be taken at this stage. Some molds have to be removed only in certain directions to avoid putting stress on raised parts of the specimen.  If this is the case, it should be noted in some way on the mold or on the jacket; it is very difficult to remember the exact shape of a bone under a layer of rubber. In general, the idea is to loosen the rubber from the specimen, and allow it to pop off.  Small specimens can be loosened and the rubber pulled down slightly to allow the specimen to push itself out of the mold. 

For two- (or more) part molds, demold one part at a time, allowing the other parts to remain supported by the jacket until you are ready to demold them. Sometimes multi-part molds will need to be demolded in a certain order.  Again, this can be noted on the jacket. 

If the specimen breaks, don't panic.  Some breaks can be glued together with Paraloid B72 or Butvar B76 in the mold. The adhesive will not stick to the mold. Otherwise, carefully remove all pieces and place them in a clean box. Remove the entire specimen from the mold. Carefully look over the specimen for any other breaks or loose pieces. Reassemble any breaks without adhesive first, as it is sometimes necessary to put one piece back first before others can be joined.  Also some pieces may fit in a way that blocks others. By fitting the pieces together dry you can decide how to proceed. Both Paraloid B-72 and Butvar B76 are reversible in case pieces need to be moved. For more information see the section of this site on Adhesives and Consolidants.

After the specimen has been safely removed from the jacket, it is important to clean it right away before returning it to the collection. 

• Paraloid or Butvar can be removed with alcohol or acetone
• Carbowax can be removed manually with a needle, and a brush with water will remove the remainder. For specimens in water soluble matrix, alcohol can be used. 
• Cyclododecane can be more quickly sublimated using heat and forced air
• Clay and wax must be gently removed with a needle, and cleaned with a brush and either alcohol or acetone. 

Make sure the number and any other pertinent information is in place on the specimen and has not been removed or rendered illegible during molding or cleaning. The specimen should be in as good or better shape after molding as before. 

For more detail on procedures for jacket making, demolding, and cleaning, download the Basics of Moldmaking by Marilyn Fox, preparator at the Peabody Museum of Natural History, Yale University.

Specimen casting can be done in either epoxy resin, polyurethane resin, polyester resin,or plaster. Choosing the best casting material for a particular project depends upon many factors.

• Resins - Resins can be colored and capture more detail than plaster, but all resins require access to a fume hood for safe use. 
• Epoxy – is often chosen to cast small fossils requiring extremely high resolution. 
• Polyurethane – is often used to create light-weight medium-large molds.  It cures quickly and produces little heat while curing thereby causing less degradation of the mold.
• Polyester – is a good choice for larger casts.  It is cheaper than either epoxies or urethanes. 
• Plaster – Plaster is easy to cast but may not capture as much detail.  Plaster dried to an opaque white.  While pigments can be added, it may be difficult to color plaster during mixing. 

Before any casting takes place, the mold should be well cleaned with alcohol, using a clean lint free rag.  The cleaning removes any traces of separator and any dirt left from the specimen. The mold should then be left for 1-2 days for the rubber to completely cure. When using a silicone or polyurethane mold no special mold preparation is necessary, but latex molds should be sprayed with mold release before use.

Plaster is easier to cast than polyester resin, but will not have the quality of detail that a resin can capture. In addition, plaster is opaque and lacks the slightly translucent appearance of bone, which makes it less suitable for display purposes. Because it is difficult to color plaster before mixing, it is better to use a white cast and paint it in the desired manner later. To keep the paint from soaking into the cast two or three thin coats of Butvar should be painted onto the cast and allowed to dry completely before painting.

Plaster is made by adding powdered plaster to water. The mix should not be too thick, about the consistency of melted ice cream. The cast is made by painting a thin coat of plaster into the mold, then adding more plaster on top of the first layer while it is still wet, to a depth of approximately 1/16".Once this layer has hardened, more plaster can be added until the cast has reached the desired thickness. The walls of the cast should be thick enough to be sturdy. Burlap or fiberglass can be also used to strengthen a plaster cast.

Once the plaster has completely dried, the cast can be removed from the mold. The mold should then be cleaned with water and the cast trimmed and cleaned up using sandpaper or an X-acto knife.

Today, many molds are cast in resin due to its strength and adaptable working properties. Resins can also be colored with powdered pigments, oil paint, or resin colors to almost any color, making them very useful for reproducing fossils. A neutral, medium grey is the useful for making scientific research casts.  This color shows the detail well and photographs well also.

All resins and catalysts are noxious materials. Always work in the fume hood and wear nitrile rubber gloves and an apron.  Avoid breathing in the dust from hardened resin and also talc or Cab-O-Sil powders. Resins should be stored in the original container and kept in a flammable liquid storage cabinet. Keep the catalyst separate. Read more on Health and Safety when using these materials.

Polyester resin for most large molds will be painted on in layers, allowing one layer to gel before proceeding with the next. In some smaller molds one layer may be enough to fill the entire mold. Polyester generates heat during the chemical reaction which makes it set. Because of this, If the material is too thick the heat generated can cause the cast to shrink, or in extreme cases can cause the cast to crack. For this reason, resin should not be more then 1/4 -1/2" thick for each layer. The number of layers needed depends on the size of the mold. Very large molds need the addition of fiberglass strand or mat for extra strength.

Polyurethane resin is usually cast solid, by pouring the resin into the mold through a hole left in the mold, known as a sprue. It can also be painted on in layers. Setting times vary with the product but are generally much quicker than either polyester resin or epoxy. The sprue and any flashing from the edge of the mold are removed after demolding. Because there is little odor associated with this product, one might be tempted to assume that the material is less hazardous to use, but like all resins it should be used and cleaned up within a fume hood.

For detailed procedures for casting in both polyester and plaster download the Basics of Moldmaking by Marilyn Fox, preparator at the Peabody Museum of Natural History, Yale University.