Re: bleaching printed cotton

[Season_Tse@xxxxxxxxx]

Dear Jenny,

I don't have any experience bleaching with perborate, but have some experience
(mainly research) with bleaching paper and cellulosic textiles, with using
stabilized peroxides and sodium borohydrides.  A very useful review on bleaching
is one written by Helen Burgess, "Practical Considerations for Conservation
Bleaching", IIC-CG, Vol. 13, 1988, pp.11-26 (also available from CCI).  There
have been a few updates on our understanding in peroxide bleaching since then,
but by and large the article covers a lot of the essentials of bleaching paper
and cellulosic textiles such as cotton and linen.

The question of colourfastness in bleaching solutions is complex and I believe
that there are no simple ways of predicting other than spot testing.  In any
case, overall bleaching of any artifacts with colourants is so high risk that it
is not recommended, however, local bleaching may be possible.

The concern about metals in bleaching solutions is mainly related to the
decomposition of the bleach in the presence of metal ions.  In the case of
peroxide bleaches, such decomposition is believed to be partly responsible for
the degradative effects often associated with oxidizing bleaches.  The other
negative effect of metal catalyzed decomposition is the reduce effectiveness of
the bleach.

As far as the pH of the peroxide bleach is concerned, the bleaching efficiency
is related to the pH of the solution, the higher the pH the more efficient it
is, but it is also more degradative to the cellulose.  So for cellulosic
textiles or paper, a safe, albeit slower acting but more controlled, pH would be
8.0-8.5.  After using a peroxide bleach, or any oxidizing bleach, it is
recommended that the artifact be immersed in a some kind of ''anti-chlor'' - a
mild reducing agent - such as sodium thiosulphate (2%; pH 7), to stop the
oxidation reaction.

For many conservators who are not required to have their artifact whiter than
white, sodium borohydride reducing bleach is often the bleach of choice.  It has
the benefit of reducing certain stains (doesn't work with all stains) at the
same time stabilizing the cellulose.  Many paper conservators prefers this
bleach because the paper retains a creamy colour, though it does have a more of
the green/blue tone, after the reddish/yellow coloured oxidized products are
removed.  Some people have found that the stained area turned slightly grey
because of selective reduction of the stain.

Sodium borohydride can be used in water or in ethanol.  The aqueous solution of
sodium borohydride self-buffers to pH 9-10, so it is not recommended for
artifacts with pH sensitive colorants or very fragile/weak or highly oxidized
textiles or paper.  In these cases, an ethanol solution of sodium borohydride
can be used, followed by thorough rinsing with water.  A common concentration is
0.5-2% (2% is the upper limit), for more control use a lower concentration.  But
it does work slower than oxidizing bleaches.  And if an ethanol solution is
used, often the lightening of the artifact is not evident until after the water
rinse, so there is a lot more potential of over-bleaching.

This is just scratching the surface.  Hope it helps,
Season


Season Tse
Conservation Scientist/Chemist
Conservation Processes and Materials Research
Canadian Conservation Institute
Dept. of Canadian Heritage
1030 Innes Rd.
Ottawa, Ontario
K1A 0M5
Tel. (613) 998-3721 (x-187)
Fax. (613) 998-4721
email:    season_tse@xxxxxxxxx

Visit the CCI Web site at http://www.cci-icc.gc.ca/




Jenny Barnett <andelos@xxxxxxxxx> on 02/17/2002 12:19:34 PM

Please respond to texcons@xxxxxxxxxxxx

To:   "texcons@xxxxxxxxxxxx" <texcons@xxxxxxxxxxxx>
cc:    (bcc: Season Tse/HullOttawa/PCH/CA)
Subject:  bleaching printed cotton



Dear colleagues,
Here is question concerning the possible bleaching of a Japanese print
on a cotton textile, probably early 20th C.. The textile is badly
stained brown by close contact with a strawboard backing in a glazed
frame. It has been washed but this was not effective. The staining is
also uneven and blotchy. In this state, it is not worth reframing it, so
the owner has agreed to a bleaching treatment. We are willing to take
the risk!

Having limited bleaching experience, I consulted quite a lot of
literature and came to the conclusion that bleaching with sodium
perborate as described in Timar-Balazsy & Eastop's "Chemical Principles
in Textile Conservation", page 231, would seem to be the best method.
This is because some of the printing inks may contain metal and a slow
process would be desirable to moniter the process.

This is how the method is described in the book:
"As a bleaching agent, sodium perborate decomposes to hydrogen peroxide
with a low reaction rate in the absence of catalysts.
A 10 g/l solution of sodium perborate buffered with a solution of boric
acid/borax buffer to pH 7 can be effective for bleaching.

I checked this out with a paper scientist and he found the pH 7
strangely low, pehaps even too low to be effective. Also, we could not
work out what the correct amounts of boric acid/borax should be in such
a solution.

The questions are:
Does anyone have experience bleaching Japanese printed textiles?
What are the correct amounts of boric acid and borax for this buffer
solution recipe?
Are there metals in black, blue, violet and light brown Japanese
printing inks?
Or is my concern about metal in the inks unnecessary?
Have I misunderstood anything?

I will certainly report back on the results of this query and the
eventual bleaching treatment.

All suggestions will be gratefully received,

Jenny Barnett
Andelos Textielrestauratie
Oude Looiersstraat 65-67
1016 VH Amsterdam
NETHERLANDS
tel/fax 00 31 (0)20 427 18 27
andelos@xxxxxxxxx