[can-talk] Tannic acid

As some of the readers of these posts will not have done any conservation training I thought I would sound a few notes of caution about tannic acid.  And I fully endorse Alison's suggestion to ask a conservator first.

 

The following are some safety issues - these would have been covered in the WA course.

  a.. Tannic Acid is a possible carcinogen according to the Materials Safety Data Sheet approved by Worksafe http://www.proscitech.com.au/cataloguex/msds/c081.pdf 
  b.. This MSDS also notes the need to wear a respirator (not just a paper mask) and chemical resistant gloves 
  c.. Methylated spirits is a solvent and is also hazardous see http://www.acsrotech.com.au/msds/ACS_Rotech_MSDS_Methylated_Spirits.pdf   Some of you are going to be saying - "I've been using metho all my life and I'm fine."  And it is true that small amounts may cause no detectable damage.  However just because it is alcohol and therefore familiar does not mean it is not harmful.  Treating big items, large numbers of items or using it on a daily basis or in a confined space will expose the user and those around them to potentially hazardous amounts.  After a while a user will no longer be able to smell the metho and can easily expose themselves to too much.  The fumes may travel onto the street or into a building, or can be absorbed into clothing and rubbish, thus exposing others who are more vulnerable such as pregnant women or children.  A careless approach to safety benefits no one.
There are also some conservation issues to consider when using tannic acid:

  a.. It turns orange rust a blue-black colour.  Was the object ever this colour?  If you had something in your collection that was originally green would you paint it black?  Tannic acid has the same effect. 
  b.. Some metals were originally blued or oiled - tannic acid does not re-create these effects and instead gives the objects a colour they never had. 
  c.. If the place where the item is kept is damp, exposed or salty the object will re-corrode.  It is far more productive to put time into improving the environment of a collection than turning it all black. 
  d.. If the rust is thick or compact the tannic acid may not penetrate fully and corrosion could continue underneath. 
  e.. A coating such as wax will provide some protection but needs to be very carefully and thoroughly applied and regularly maintained.  Does your organisation have the resources to inspect all the treated objects annually?  And wax makes things dark and shiny too. 
  f.. Is the rust actually active?  Some rust is quite stable and can tell an interesting story of the history of the object.  Do we want everything in the collection to look brand new?  Bright shiny black objects may look nice and tidy but are they real?  Did they ever look like this when in use?  Farm equipment that has been left to rust in a paddock tells a sad story of its own about changes in farming practices over time, this story could be told in the interpretation. 
  g.. If the corrosion (rust) is not active or is proceeding slowly then applying tannic acid is a purely cosmetic measure and would seem to be a low priority in the busy lives of most community museum volunteers. 
  h.. As an example, no coating has been applied to the Statue of Liberty despite it being exposed to a very salty and polluted environment.  Measurements of the rate of corrosion showed that it would not perforate for 1000+ years. No coating would survive that long and the costs of maintenance of any coating were too high. 
  i.. Many old rusty iron and other metal objects are just as important as the other items in a collection and deserve careful management and professional treatment. 
  j.. A Significance Assessment coupled with a Preservation Needs assessment would help identify which items are both significant and in urgent need of treatment - thus helping custodians prioritise. 
  k.. Good significance assessments should also give information about the original appearance of items.
  l.. A conservator would always undertake a condition report and prepare a treatment proposal before embarking on any treatment.  This would include identifying what sort of corrosion was present, how active it is and what was causing it and determining whether tannic acid is actually the best option. 
  m.. Preventive conservation - dealing with damp, building sheds and shelters and regular dusting will do far more to preserve many iron and steel collections.
Jenny Dickens

Conservator

  ----- Original Message ----- 
  From: alison.wain at anu.edu.au 
  To: can-talk at collectionsaustralia.net 
  Sent: Wednesday, July 09, 2008 11:20 AM
  Subject: [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



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