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Starch Pastes, etc.



The thread(s) on paste, methyl cellulose, thymol, and microwave paste
exemplifies good and less good aspects of a listserv.  On the one hand,
people are sharing information and that is good.

On the other hand, what is being shared is largely empirical and that can
make it difficult to interpret.  For instance, thymol can be dangerous to
people, but, unless I'm mistaken, it is still used in Listerine mouthwash.

Microwaves will make a paste from starches, but it is not as strong as a
cooked paste.  When I have compared them under a microscope, microwave
paste still has granular starch particles and cooked paste does not.

Methyl cellulose is available in a variety of molecular weights, and some
are better than others for adhering paper and similar material.  The Dow
Chemical Company (and others) have technical data sheets which give this
information, as well as directions for preparation and recommended use.

There are many sources of information about the properties of starches, and
one which is fairly readily available is Irving Skeist's _Handbook of
Adhesives_

The important thing is to gelatinize the starch granules during the cooking
process.  My basic recipe for making wheat starch paste is included in the
article "Art-on-Paper" which is available on my web page.  Essentially, I
stir the paste as it is heating up until it is done.  I have two ways to
test this.  First, the paste will become thick as the starch granules
absorb as much water as they can, and then the membranes will begun
bursting (gelatinization); at this time the paste thins out slightly and I
continue stirring for a minute or so.

The second test occurs when I pour the paste into a container of cold tap
water.  If the paste dissolves and clouds the water, it was not done.  One
caveat.  When I was making paste in Ft. Collins, Colorado (about a mile
above sea level) there was a little bit of cloudiness in the water, but
that came from the small granules of starch which I was not able to heat
enough due to the lower atmospheric pressure which made the water in the
double boiler boil at a lower temperature than what I am accustomed to in
Portland, Oregon, near sea level.

A properly made and applied wheat starch paste is not very reversible,
contrary to what some people have said.  It may or may not be acidic,
depending on how it was manufactured.

Hope this helps.

Jack

Jack C. Thompson

Thompson Conservation Lab.
7549 N. Fenwick                               I hear and I forget,
Portland, Oregon  97217                       I see and I remember,
                                              I do and I understand.
http://www.teleport.com/~tcl/


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