[can-talk] Tannic acid

Dear John,

The recipe for a tannic acid solution provided by the Canadian Conservation Institute is

100g tannic acid (this is bulky - use a 2 litre mixing vessel)
900ml deionized or distilled water
50 ml ethanol (methylated spirits will do)
2ml of dilute phosphoric acid (H3PO4) - this is optional but improves the reaction process

You don't need to worry about using a water based solution - the water will evaporate as the rust is converted to ferric tannate which is the protective layer you are after. A couple of things to note - this doesn't work well on clean metal - it is the iron oxide (rust) which is converted, not the unrusty iron. Also, you need to use several thin coats and allow them to dry in between - it is the reaction of the solution with the iron oxide in air which matters. Also, CCI says you don't need to coat the surface afterwards, but I have always found the tannic acid coating really needs to be protected after treatment by a coating of wax - it's not a good long term protective on its own.

If you don't have experience in using a compound like this, it is a good idea to make contact with a conservator in a museum in your area and discuss what you want to do with them. Knowing exactly  how and when to use the compound may save you a lot of time and hassle. You can also buy the CCI notes for this and many other useful treatments online - they are really good and give clear, step-by-step instructions.

Alison Wain
Research School of Humanities
Australian National University

-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.collectionsaustralia.net/pipermail/can-talk/attachments/20080709/ec96c05f/attachment.html