Acid preparation of fossils using sulfamic acid, a weak organic acid, and its advantages over acetic and

Abstract

Aparece como "oral comunication" en la web de la ACID PREPARATION OF FOSSILS USING SULFAMIC ACID, A WEAK ORGANIC ACID, AND ITS ADVANTAGES OVER ACETIC AND FORMIC ACID PREPARATION — PADILLA, Carlos, Fundacion Colombiana de Geobiologia, Bogota, Colombia; PARRA, Mary, Fundacion Colombiana de Geobiologia, Bogota, Colombia Organic acids such as formic and acetic have long been used for preparation of fossils embedded in calcareous matrices. Formic acid is considered preferable to acetic acid as, although it is more aggressive, it less damaging for the fossil to have fewer immersions to remove matrix. However, at the Fundacion Colombiana de Geobiologia we have been experimenting with sulfamic acid in order to determine its effectiveness and to optimize its usage. Sulfamic acid (H2NSO3H) is a weak organic acid that is less toxic than acetic or formic acids. It has been used extensively in domestic and industrial cleaning products to remove limescale. Sulfamic acid’s logarithmic dissociation constant (pKa) is 1.18 at 25° making it stronger than acetic (pKa 4.76) or formic (pKa 3.74) acids. Due to its greater acidity, it more effectively removes large volumes of matrix given the same number of immersion cycles. The procedure developed follows the general outline of other acid preparation techniques. By experiment, 2% weight by volume (wbv) acid concentration was found to provide a good balance between time, number of immersions and fossil integrity. The fossil goes through a preliminary cleaning cycle of 10 minutes in 2% wbv sulfamic acid followed by a one day rinsing and drying cycle prior to initial protection of the exposed bone. The specimen is washed in ethanol to displace remaining moisture, and then 5 % wbv Paraloid B-72 (methacrylate resin) in ethanol is applied to the fossil as a first protective coat. Further coats of 15% wbv Paraloid are applied and this is repeated until a homogeneous, non-porous film is achieved. The specimen is completely dried before being submitted to 2% wbv sulfamic acid buffered with calcium phosphate (Ca(POH)2) to inhibit chemical action to exposed bone. The first immersion is carefully observed to see the extent of matrix removal. Results indicate sulfamic acid is effective for removal of calcareous matrix surrounding large fossils, which would otherwise have required many more cycles in acetic or formic acid. The fossil material preserves the same fine detail exposed with acetic and formic acids, but with less stress to the fossil.