2 EMBEDDING MATERIALS

An ideal embedding resin should meet the requirements listed below when used for the analysis of organic components in paint cross sections obtained from works of art. Different criteria are used when samples are embedded for other techniques such as petrography (Reedy 1994) or SEM analysis.

1. The mounting medium and its solvent should not react with, soak into, or otherwise interfere with the analysis of the binder in the sample.
2. The medium should cure at room temperature. The curing process should not be exothermic, since heat may adversely affect the organic materials or binders in the sample.
3. For samples that are to be analyzed by infrared microspectroscopy, the resulting embedment and sample should be easy to section using a microtome. Transmitted infrared analysis requires a section of 1–10 μm in thickness for an optimum spectrum.
4. The medium should be transparent to ensure proper orientation of the sample for microscopic examinations as well as for positioning the embedded specimen in the microtome jaws.
5. The medium should not shrink upon curing, because shrinkage will apply stress to the sample and may cause problems during microtoming.

The advantages and disadvantages for several types of polymeric materials tested for embedding paint cross sections are summarized in table 1. Figure 1 shows example blocks of each of the polymeric media listed in table 1. Many other types and brands of embedding media are available; some are discussed in the text. Of the four major types of embedding resins tested, polyester was found to be the best for embedding and microtoming most types of art materials.

TABLE 1 TYPES AND BRAND NAMES OF POLYMERIC MEDIA TESTED FOR EMBEDDING AND MICROTOMING OF PAINT CROSS SECTIONS

Fig. 1. Example blocks of polyester, epoxy, acrylic, and wax embedding media for paint cross sections corresponding to table 1. Break-away molds and small, flexible silicone rubber molds are shown.

One polyester embedding resin commonly used in art conservation is Bio-Plastic. Similar polyester resins are sold under the brand names of Caroplastic, Castolite, Castoglas, and Vestopal W (see Suppliers). Polyester embedding resins contain a polyester prepolymer dissolved in styrene monomer to form a solution of appropriate viscosity (Horie 1987). When combined with a methyl ethyl ketone peroxide catalyst, a crystal clear, uniform, solid block is produced that is readily polished and easily microtomed. The resin cures overnight at room temperature or in a few hours at slightly elevated temperatures. It shrinks minimally during curing at room temperature and slightly more when cured with increased temperatures. It is cheap, readily available, easily prepared, and only moderately toxic. These many positive features give polyester resin several advantages over the other embedding resins on the market (e.g., acrylics, and epoxies) and account for its popularity as an embedding medium for paint cross sections.

While the polyester resins typically are used for paint cross sections in art conservation, forensic scientists generally use epoxies, acrylics, and cyanoacrylates. We found that the epoxies tend to be yellow in color as well as too hard and brittle to microtome sections at the thicknesses of 1–10 μm required for infrared analyses. The acrylics were clear and colorless, but they were very exothermic and shrank significantly upon curing. One acrylic embedding material became so hot during curing that it melted the 1 in. plastic mold. Both the epoxies and acrylics had dissolution and infiltration problems similar to those of the polyesters and thus are not recommended over the polyesters.

One method found in forensic science is the use of a drop of cyanoacrylate glue (e.g., Super Glue or Krazy Glue) to mount a paint chip on the end of a small wooden dowel (Cartwright et al. 1977). While it was difficult to get the sample oriented correctly, this method worked well for microtoming the sample. However, we still experienced infiltration problems with some samples, and dissolution of components is still a potential problem when the sample contains acrylic paints, waxes, or fresh resin layers. Krazy Glue gel, a thickened cyanoacrylate specifically designed to remain on the surface of porous substrates, also soaked into the outer layer of all plaster samples we tested.

Gelatin is another material that has been used successfully in the forensic field for embedding paint cross sections (Wilkinson et al. 1988). The procedure used by Wilkinson involved freezing the sample in a gelatin block for cryogenic microtoming. Afterward, the thin section was warmed and the gelatin was washed away, leaving just the thin section of sample. This method should be effective for samples that are not susceptible to water, such as wax cross sections. However, we did not test this method in our lab because gelatin could potentially interfere with the positive identification of glue binder, albumin varnish, or other protein-aceous media. In addition, glue or gum binders in the samples may be susceptible to damage or alteration by the water solvent in the gelatin.

The biomedical field uses several types of embedding media (e.g., glycol methacrylate, methyl cellulose, acrylamide, and epoxy) for cytology studies by electron microscopy. These studies often require ultrathin microtomy for producing thin sections of less than 1 μm thick. A low-viscosity embedding solution, usually water-miscible, is used to allow easy penetration of the resin into the sample. For the biomedical field and for many other purposes, penetration is desirable since it will stabilize the sample and help maintain the specimen's shape. However, since we were trying to eliminate infiltration of the resin, we did not test any of these low-viscosity media.

One embedding option is the use of low melting-point waxes such as Paraplast. This wax-polymer mixture melts at approximately 60�C and solidifies rapidly. The wax does not pose any infiltration or dissolution problem to the samples, but it is unlikely that surface wax layers would be detectable. While wax embedments are too soft to be polished, they can be microtomed easily. Wax is opaque, so it is critical to know the positioning of the sample before microtoming. It is also important to work quickly when pouring a mold with the hot wax because delays can result in a block that contains many bubbles. One method that shows potential has been developed by (Wolbers 1993). A wax is mixed with an inorganic salt, such as potassium bromide, thus increasing the hardness of the wax and making the mixture more transparent.

Similar to waxes are hot-melt adhesives, which are primarily polyethylene-based polymers. These adhesives range in transparency from semiclear to opaque. They are hard and rubbery at room temperature. Hot-melt adhesives are not recommended for general embedding purposes because they solidify rapidly (less than 60 seconds), making it difficult to orient and manipulate the sample. These adhesives did not infiltrate the samples, but they were slightly too soft for easy microtoming and had to be cooled in order to obtain thin sections. Optimally, this process requires a cryogenic microtome, since cooling samples prior to microtoming in a freezer or with a cryogenic spray can result in water condensation on the sample.

Several silicone rubbers have been tried for embedding samples. To date, we have not found any that are hard enough to be microtomed or polished. However, we are still looking for other silicone materials that may work.

Since the use of nonpolymeric mounting media would eliminate the dissolution and infiltration problems associated with the poly-meric media, we tested several inert materials, such as salts (BaF2, AgCl, KBr), cork, indium, and gallium. The inorganic salts barium fluoride, silver chloride, and potassium bromide were powdered and then placed in a pellet die with the sample. The salt was pressed into a transparent pellet with no apparent distortion to the sample. At this point, it was difficult to proceed further. We were not able to micro-tome any of the pressed pellets without the salt crumbling into a powder. And while the salts themselves could be easily polished, they did not polish at the same rate as the sample and tended to disintegrate.

Problems also occurred with cork embedments. A small slit was made in a piece of cork with a razor blade, the sample was inserted, then the sample area was sealed with cyanoacrylate adhesive (such as Krazy Glue). The cork itself was difficult to microtome because it was so soft. It tended to move and compress with each microtome slice. However, the area of cork and sample coated with cyanoacrylate exhibited some additional stiffness that allowed it to be microtomed into sections approximately 10 μm thick. We were not able to polish the sample, and it was necessary to use a fairly large sample (1 � 2 mm) in order to position it correctly. The same dissolution problems mentioned above for cyanoacrylate would still apply.

Malleable metals, gallium and indium were tried for embedding. In this test, two small pieces of metal were placed above and below the sample. Then some pressure was applied to compress the metal around the sample. The metals held the samples well for microtoming, but it was difficult to orient the samples properly because of the opacity of the metals. Since the metals are also expensive and toxic, this method was discarded.

Because an alternate embedding material was not found that had as many advantages as the polyesters, we focused our efforts on understanding the interaction of the polyester resin with the sample in hopes of finding conditions, samples, and methods for which the polyester works well.