Re: Chelating agents for rust stain removal

[Season Tse <Season_Tse@xxxxxxxxx>]

Hi Carol,

There are a number of chelating agents and rust removers that would be
effective in removing rust stains from cotton or linen.  The choice depends
on:
1.    the vulnerability of the textile - how fragile or robust is it, are
there pH sensitive dyes
2.    whether the textile can be immersed and for how long
3.    for local stain removal, do you have the use of a textile suction
table/disk with high enough suction that would prevent lateral migration

The method of choice for many conservators is dithionite (5%) follow by a
chelating agent either 5% EDTA or DTPA (diethylenetriamine pentaacetic
acid).  The dithionite/EDTA method require a certain amount of soaking -
needs time - to work.  Immersion is needed for treatment and for rinsing.
The benefit is that the the initial pH of sodium dithionite is neutral and
is preferred for cellulosic materials.  With time the dithionite solution
oxidizes and slowly become acidic.  As soon as the solution turn slight
yellow it is a good idea to make up a fresh solution.  A fumehood is needed
for this treatment because of the smell.  The two step method (dithionite
first, follow by DTPA or EDTA) was found to be more effective in reducing
colour reversion of the rust stain due to re-oxidation.  Because this
method takes time to work, for local stain removal, using a suction table,
this method really doesn't work well.  Bear in mind that dithionite is also
a mild bleach, and would brighten white textiles but also alter the colour
of some dyed textiles.

Ammonium citrate and citric acid have also been reported to be effective.
But like dithionite, it would likely take time to remove stain, especially
for stubborn stains.  Thorough neutralization and rinsing is always needed.

More aggressive rust removers are the commerically available products that
some dry-cleaners used, 12% hydrofluoric acid or 5% oxalic acid.  The
downside of these is obviously their acidity, though HF theoretically do
not affect organic material.   The advantage is also the speed at which
these reagents are able remove (chelate and solubilize) rust stains.  So if
the contact time is very short, eg. on a suction table, and if
neutralization and rinsing is very thorough, they are potentially useful.
We are looking into these rust removers and are studying their effect on
cotton, linen and silk textiles, to see if there is a way to use these
safely.  These would potentially be used for local stain removal where
total immersion in systems such as dithionite/DTPA is not possible.

One final alternative that we are exploring, in the case that there is no
safe way of removing the stain, but would like to stabilize the textile
from iron catalyzed deterioration, is the use of calcium phytate, a
chelating agent that binds up the iron preventing it from catalyzing
oxidation.  The reagent however doesn't remove the iron.  It has been
proven to be effective in arresting iron gall ink corrosion on paper.

Season
Season Tse
Senior Conservation Scientist/Chemist
Conservation Processes and Materials Research
Canadian Conservation Institute
1030 Innes Rd.
Ottawa, ON
Canada
K1A 0M5
Tel: (613) 998-3721 x-187
Fax:(613) 998-4721
email: season_tse@xxxxxxxxx
Websites: www.cci-icc.gc.ca  &  www.preservation.gc.ca



                      Amann conservation
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                      VATION.COM>                cc:
                      Sent by: Textile           Subject:  Chelating agents for rust stain removal
                      Conservators
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                      10/03/2004 05:18 PM
                      Please respond to
                      Textile
                      Conservators






Hello all,

My colleagues and I are looking for information on the use, or research on
the use of chelating agents in the removal of rust stains on cotton or
linen textiles.  We are sure that we have seen something on some dist list
or another but our computers crashed recently and we lost all e-mail
records we had saved.  Any info would be greatly appreciated!

Thanks,
Carol Kropnick
Amann Conservation