EMBEDDING PAINT CROSS-SECTION SAMPLES IN POLYESTER RESINS: PROBLEMS AND SOLUTIONS
MICHELE DERRICK, LUIZ SOUZA, TANYA KIESLICH, HENRY FLORSHEIM, & DUSAN STULIK
In a comparison of several types of polymers, polyester resins met the requirements for an ideal embedding media for most paint cross sections much better than did epoxies or acrylics. Polyester resins set rapidly without heat, are clear, polish readily, and microtome easily.
However, embedding resins, such as polyester, can dissolve some waxes, colorants, and resins. In addition, during the embedding process, porous, low-binder paints are penetrated by most types of polymer solutions, including polyester. Infiltration can occur with matte or porous paints that are often found in wall paintings, polychrome sculptures, ethnographic objects, and with glue gessos. This infiltration is visually detected by examining the cross section for poorly defined edges, darkening or discoloration, and blotchy stain results. Infiltration may be confirmed by the analysis of the sample with infrared microspectroscopy, where the presence of the embedding resin in the sample will produce a spectrum containing absorption bands characteristic of the resin.
It is important to recognize that infiltration can and has occurred in embedded paint cross sections. Infiltration has advantages and disadvantages, because it consolidates and supports the sample but can also hinder the analysis of the sample's components. Thus, potential infiltration should be considered when samples are examined, and a decision should be made whether it is necessary to prevent the infiltration based on the type of sample and the type of analyses.
In the cases where embedding without infiltration is chosen as the best option, several barrier methods have been tested for coating the sample with materials before embedding the sample in polyester resin. A method using Rhoplex AC-33 thickened with fumed silica worked best, forming a uniform coating around the sample that, when dry, did not allow the polyester resin to infiltrate into the sample. Many other types of high viscosity or thixotropic materials would probably work as well for precoating the sample.