Paste preparation for works on paper

Lanora Pierce <lanorapierce [at] gmail__com> writes

>Does anyone have an opinion on which is a better preparation method
>for rice or wheat starch paste when comparing the cook and stir
>verses the microwave recipes? The microwave methods don't call for
>soaking the paste for an hour first, and I'm curious as to why and
>if there is something about the microwave that makes any difference?
>The paste is used for repairing and/or mounting works on paper.

In general it is not advisable to use rice starch since it yellows
easily and becomes brittle. In Japan it is used for rigid joints
such as in shoji. Also, wheat starch contains more amylose, which is
the part of starch that will become sticky on cooking. Wheat starch
is therefore the conservator's weapon of choice.

By soaking the starch granules in water for some time (1 part to
four parts water, up to 12 hours--overnight) water is allowed to
penetrate between the starch layers in the granules. This will
result in a softer, more easy workable paste once cooked. The
traditional cook and stir method allows for shear forces in the
paste once the granules start to burst open after the temperature
reached +/- 85 deg C. Prolonged cooking and stirring (shear forces)
is necessary in order to break all the starch layers and thus
maximise the gelatinization of the amylose and amylopectin.

The longer cooking will degrade some of the paste. This is not
considered to be a problem and causes the breakdown of the larger
starch conglomerates. In this way a larger spread from small to
larger conglomerates in the paste will occur. Working the paste
after cooling and setting will result in a smaller internal surface
of the paste because small particles will fill all the spaces
between the larger conglomerates. In this way the paste will have a
higher contact ratio with the paper thus making more hydrogen bonds
possible between paper and paste per area. This will ideally result
in a better bond and is responsible for the high dilutions
attainable with well cooked and prepared wheat starch paste.

Conclusion: microwave paste is handy and easy to make but normally
will make lower quality, harder and under-cooked paste. The easiest
way of making paste in my opinion is using a Saucier and covering
the pan to achieve a higher cooking temperature. The constant
stirring in the Saucier will help in breaking up the starch
conglomerates without breaking your back.

Bas van Velzen
Senior tutor, book and paper conservation
University of Amsterdam (UvA)


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                  Conservation DistList Instance 25:39
                 Distributed: Friday, February 24, 2012
                       Message Id: cdl-25-39-001
                                  ***