Bleaching
As textiles age they often become more yellow in appearance. This is particularly true of white wools, Yellowing can be a sign of acidic deterioration products which originate from the wool. These by-products are water soluble. The removal of this yellowing can be desirable to prolong the life of the textile. Removal of the acidic by-products will not return the item to white. Aged fibres develop a rich, creamy patina — a sign of the textile's authenticity and age. To remove this patina would weaken the fibres and reduce the value of the item.
It is also important to note that the white cloth, which we are used to seeing, is a modern invention. "Whiter than white" is obtained by optical brighteners (these dyes glow under a black light) and they are a fairly new dye development. If an old item were to be cleaned to today's standards of whiteness, it would no longer appear authentic and its value would be lessened.
Never use commercial washing powders or bleaching products on historic items, as the optical brighteners will change the colour of your textile. Chemical residues from these products are damaging. In time, this new damage will cause yellowing and so the cycle continues.
Bleaching is either an oxidative or reductive process. Either can be extremely damaging to fibres. In fact, items have been known to turn green, if the wrong bleach is used over residues from the other process. On occasion a conservation bleaching may be recommended. There are several methods used:
Light bleaching using UV light. An item is immersed in water to absorb the excess UV to ensure no damaging rays affect it. This process requires skill and time. It is unsuitable for fibres which are weakened and may split on swelling with water.
Oxidative bleaching: The textile is exposed to dilute hydrogen peroxide in a moisture chamber under extremely controlled conditions.
Reductive bleaching: Sodium bora hydride is used to convert the damaged cellulose molecules to the pre-yellow state. This is a process, which can strengthen cellulose fibres. It is not suitable for wool or silk. It is not suitable for cotton items which cannot be immersed. Sodium bora hydride can't be bought without a special proper laboratory and shouldn't, as it can be explosive (combined with water) and very dangerous if not used properly.
I never use commercial or chlorine bleaching products. Bleaching would only be recommended if the staining on the item were detrimental to the item's appearance. Bleaching is carried out over several hours at extremely low concentrations and monitored for the entire time. Bleaching relies on a chemical reaction between the solution and the cloth and this makes the procedure unpredictable and risky.
Discuss the risks carefully with me so you can make an informed choice before exposing your precious textile to the dangers of bleaching.
Reprinted with permission of:
Victoria Gill
Conservator/Director
Endangered Heritage Pty Ltd
Shop 4 Duffy Place
PO BOX 7257
Duffy ACT 2611
P 02 6287 1291
F 02 6287 1209
M 04 1481 5436l
They have a retail line of archival products and an object conservator who works in metal ceramics glass etc. Saving valuable and sentimental heritage items from extinction because the next generation deserves the opportunity to cherish its past. www.endangeredheritage.com