Removing iron from artifacts and fossils
By Russ McCarty
Those of you who collect in Florida’s northern and central rivers have frequently encountered specimens that would be perfect except for the disfiguring hematitic (iron) stains and encrustations on them. In fact, in some rivers in the Florida Panhandle, this is the norm. Attempts to remove these stubborn deposits by physical methods, such as grinding or using air abrasive tools, most often results in permanent damage to the specimens regardless of their composition.
In 1974, Francis Howie, of the British Museum described a method for removing hematitic matrices from vertebrate fossils by using a dilute aqueous solution of thioglycollic acid. Howie first coated all exposed surfaces of the bone with the resin, polystyrene, to provide a protective barrier. After the resin had cured for six hours, he immersed his specimen in a 5% aqueous solution of thioglycollic acid (19 parts distilled water to 1 part thioglycollic acid). To this solution was added 0.9% calcium orthophosphate by weight. The addition of the calcium orthophosphate is to prevent the thioglycollic acid from scavenging phosphate from the bone. Howie left his specimens in acid for 24 hours. They were then removed, allowed to drain, and then immersed in a 5% ammonium hydroxide (ammonia) solution to neutralize the acid. The specimens were then washed in several changes of water for four days. He then used gentle brushing, and air abrasive to finish removing the deposits.
As you can see, even this “easy” method is fairly time consuming and meticulous. But for an important specimen, the time spent in chemical development would be repaid. My first experiment on removing iron deposits with thioglycollic acid was on a chert tool from the Aucilla River. The chert specimen was so heavily encrusted with iron deposits that it was hard to discern the true nature or shape of the tool.
I placed the tool in a 5% solution of thioglycollic acid made up as above—except that I left out the calcium orthophosphate. I felt that this was an unnecessary buffer since there was no bone involved. The specimen was left in the acid solution for 48 hours. It was then removed, rinsed in water, and placed in a 5% ammonium hydroxide solution for a few minutes to neutralize the acid. At this point the specimen was placed in a water bath for several hours. The iron deposits had turned to a soft, powdery film that brushed away easily with a soft toothbrush. All traces of the iron deposit were gone.
Thioglycollic acid is available from Fisher Scientific supplies at a cost of about $25 dollars per 100 ml bottle. That would make about 2000 ml of 5% solution. When working with any chemicals, one should always read the materials safety data sheets which the suppliers provide and follow all rules for safe usage. Thioglycollic acid produces hydrogen sulfide as it digests the iron compounds. Hydrogen sulfide is a poisonous gas, but familiar to everyone as the smell of rotten eggs and sewer gas. In the small quantities used for removing iron from flint tools, the use of this acid should present few problems, especially if the process is performed in a fumehood or in a covered container placed in a well ventilated room.
Howie’s method (Howie, F.M.P. 1974. Introduction of thioglycollic acid in preparation of vertebrate fossils. Curator 17:159-166) has proven successful for many applications and should be reviewed before attempting to remove iron from fossil specimens. Using thioglycollic acid, while applicable to many situations, does have a few drawbacks such as the foul odor produced by the acid. An alternative noncorrosive method which uses no acid was developed by Rob Waller, a Canadian conservationist, and adapted to vertebrate fossils by Blum, Maisey, and Rutzky (Blum, S.D., J.G. Maisey, and I.S. Rutzky. 1989. A method for chemical reduction and removal of ferric iron applied to vertebrate fossils. Journal of Vertebrate Paleontology. 9(1):119-121. This method is a bit more complicated than Howie’s method.