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Hi there! This might be a slightly esoteric question, but I was curious about the use of starch paste as an adhesive for use in paper artifact conservation.
Why are wheat and rice starches favoured over other types of starch, for example corn or potato starch?
Is it due to the ageing properties of the starch itself (I'm thinking for example of how simple wheat flour paste ages poorly due to the gluten content, though this presumably would not be the case with corn or potato, even in unpurified form)?
Or is it other qualities of the adhesive, such as reversability, cost, ease of application etc?
Thanks in advance!
Move this whole section up, swapping places with the section above it.
We will send this to our paper conservation moderators and
those who focus on preventive conservation in the hopes of an answer.
This is a good question. Corn and potato starch have sometimes been used in historical applications, but you're right that rice and wheat starch paste are preferred for use in conservation. C.V. Horie notes that potato and corn starches are more likely to be damaged during the cooking necessary to prepare a paste from starch, which leads to yellowing. Wheat starch paste also has a higher proportion of amylose, which is what causes the paste to gel upon cooling. (see: Materials For Conservation: Organic Consolidants, Adhesives, and Coatings by CV Horie).
Thanks for the responses. I had not heard of starch yellowing due to overcooking. Very interesting!
much Gillian. Here is an iconoclastic, but likely really to-the-pont, response from Hugh Phibbs.
I find it hard
to not agree with him. First we must ask, why use food as an adhesive, in an
era when out climate favors increased infestation? For years I heard that
methyl cellulose was too weak to use for securing art and when I learned that
it was used successfully as a re-wettable adhesive in Europe, I
reasoned thus: MC is non polar and thus less penetrating than polar starch, but
water is very polar and if it is used to activate the dried MC film it will
penetrate and draw the MC in with it. I inferred this having observed MC
dehydrate starch, in a jar. Indeed, if one adds enough water, a dried MC film
can from a bond that is stronger than paper. However, that entails adding H2O,
a very electromotive molecule that swells organic materials, among others.
I was introduced to the idea of using Klucel G, HPC, in alcohol, at the INP in
Paris, something else I had been warned off as being too weak. When it is used
as a consolidate, it is used in a very dilute form, but if instead, it is
made thicker, as a paste, it forms a better adhesive, which also
has surfactant properties (with no surfactant added) and can adhere to
plastics. It is also non polar and non biological, so it will not
encourage infestation. Films of MC and HPC are relatively clear and much more
flexible than starch and both can be reversed more easily. Neither
requires cooking and both can be stored at room temperature. I have no
definitive recipes, for either but would recommend something like a
minority of powder of MC to a majority of H2O and the same for HPC and
isopropanol and extensive testing. I can still hear the words of a noted
textile conservator who said to me, “Launder your textiles before storage,
since food stains is where the bad stuff happens.” and thus, I am happy to avoid
biology. On a side note, I think I finally know why starch is aged for
many years in Asia, since that, by my way of understanding,
will exhaust its biological potential. Using HPC or MC is so much easier.
We need more responses like this.