Alvaro Keding/© AMNH
Preparators are always searching for materials that will enable them to do the successfully accomplish the tasks associated with revealing and preserving fossil specimens.
Most materials are borrowed from other fields and industries. Information on some of the materials commonly used in preparation and their applications are given here.
More detailed information on specific materials can be obtained from resources like the Conservation & Art Material Encyclopedia Online (CAMEO) database.
Sometimes during the process of preparing a specimen there are times when it may be necessary to support a join with a small surface area, bridge a gap for display purposes, reconstruct a section, or form a strong support for an a cantilevered area of bone. Fillers can be made from:
For detailed notes on the properties of different filling and bulking materials download the Searching For The Filler Of My Dreams – An Odyssey In Gaps And Glues by Yale Peabody Museum preparator Marilyn Fox.
Read more on Adhesives.
Filling of voids in a fossil specimen is an important step in the molding and casting process. Unlike the preparation process when fillers are used to bulk out gaps or joins and may need to permanently support parts of the specimen, when filling for molding the goal is to remove the fill completely once the mold is completed. Therefore careful selection of the fill material is important. Good fillers must be easily workable, easily removable and safe for the specimen and preparator. Some of these materials such as Carbowax and cyclododecane are also useful as temporary supports during preparation.
Properties
Carbowax is the brand name for a polyethylene glycol (PEG) product made by Dow Chemical company. Carbowax comes in a variety of viscosities and hardnesses. The product number refers to its molecular weight and 3500-4000 are often good for moldmaking, although softer or harder ‘waxes’ can be used or mixed together to form a product that is easily workable. Carbowax may also be colored with a tiny bit of powdered pigment, to make it more easily readable against the bone. The advantage of PEG products over real waxes is that they are soluble in water, making them easily removable from the specimen after molding.
Application
Carbowax can be melted using a small hot plate set at a low to medium heat. The melt is applied to the hole and when cool, scraped flat with a tool until it completely and tightly fills the hole. Smoothing can be done with a brush and acetone or alcohol. Very tiny holes can be filled by simply scraping a little of the hard wax into the hole and using acetone or alcohol to smooth it. The surface of the filled area should be slightly below the surface of the bone, to allow it to be more easily distinguished.
Properties
Cyclododecane (CDD), an inert cyclic alkane, is a relatively new material in the field of preparation. Its usefulness derives from its ability sublimate (i.e., pass from a solid directly to a gas) at room temperature, making removal efforts by the preparatory easy or even unnecessary.
Application
Cyclododecane can be applied in solvent and is available as a spray, but for filling is most commonly applied by heating on a small hot plate and applied as a melt. The wax cools extremely quickly so it can help to heat the specimen slightly, for example under a heat lamp (Of although, this may also speed the sublimation of previously filled areas). Cyclododecane can be worked like any wax with tools or solvent. For information specialized application tools such as a kitska (used by Ukrainian dyed egg creators) and the tjanting (batiking tool), as well as important information on the health and safety of CDD see the following references:
Sublimation and removal of the CDD can be sped by the application of heat or forced air, like a hair dryer on a low setting. This disappearing act makes removal of the fill much easier and safer than the manual or melting removal necessary with other materials. It must be noted however that the flip slide of this is that the CDD can potentially disappear before the mold is made if too much time is taken before the rubber is applied.
Properties
Microcrystalline wax is a high molecular weight hydrocarbon wax with a fine crystalline structure. It is easy to obtain and to work but can be time consuming to remove entirely.
Application
The wax can be applied using small hand tools and softened either with heat or mineral spirits.
Properties
Modeling clay is probably already on hand making it an easy option for filling voids but it is less easily removable after molding is complete than the products listed above. It can be easily worked and scored to ensure it is distinguishable from the original bone. The modeling clay should be sulfur-free as sulfur will inhibit the curing of silicone molding products.
Application
The small hand tools appropriate for working carbowax are also useful for modeling clay.
Application
Very deep or large apertures can be stuffed first with tissue and a thin layer of filler applied on top. On very porous bone, where the surface of the bone is missing, the entire area can be covered with a thin paper, such as “Kim Wipes” or tissue tacked down with Acryloid B72.
Application
A flexible film can be created by painting a very thin coat of a water-based glue onto Mylar or waxed paper. When dry, the adhesive can be peeled off and applied to the surface of the specimen.
Molds of fossil specimens are general done in one of three materials. For more on molding see the pages on Molding & Casting in the Studying section of this site.
A polymer that contains silicon, carbon, and oxygen. Silicone resins are made by the room-temperature vulcanization (RTV) of silicone oils. Molding silicones cure by the addition of a catalyst. The system can be either tin based or platinum based. Once cured, silicone resins are chemically inert and can exist as elastomers and resins (both thermoset and thermoplastic). They function over a wide temperature range, are water repellent and have very poor adhesion. Silicones are used in fossil preparation for molding when replication of very fine detail is important. However, silicone rubber can be easier to tear, making it less practical for extremely large molds.
Latex is a dispersion of polymer microparticles in an aqueous medium. Latex in used for molding where detail is not as important. It is more economical for large molds and it is stronger and difficult to tear.
Any polymer consisting of a chain of organic units joined by urethane (carbamate) links. Polyurethane polymers are formed through step-growth polymerization by reacting a monomer containing at least two isocyanate function groups with another monomer containing at least two hydroxyl (alcohol) groups in the presence of a catalyst
Polyurethane rubbers are strong, but curing properties may be affected by moisture in the air, and the rubber can be subject to degradation by ultraviolet light.
Choosing the best casting resin for a particular project depends upon many factors. For more on casting see the Molding & Casting pages in the Studying section of this site.
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 and less hazardous to the heath of the user than either epoxies or urethanes.
Plaster is easy to cast but may not capture as much detail. Plaster dries to an opaque white. While pigments can be added, it may be difficult to color plaster during mixing.
These Fossil Preparation resources were originally developed in 2007 with the support of the National Science Foundation (NSF).
