Many people had asked me about making starch paste since all our bookbinding workshops use copious amounts of this wonderful natural, non-toxic glue .
I have never treated starch paste making as a science, you know, like having fixed amount of water added to starch powder and how hot the temperature must be or how long to cool it, etc. Instead, it is mostly through trial and error, adding additional amounts of water on the fly and if the mixture did not turn out well, then start over again.
But I thought it will be good to share how I normally do my glue and to explain briefly the science behind starch making.
Starch is present in plants as a form of storage. It is a complex carbohydrates that has powerful thickening properties. When starch is combined with water or another liquid and heated, individual starch granules absorb the liquid and swell. This process, known as gelatinisation, is what causes the liquid to thicken.
There are various types of starch, grinded into powder form available in Singapore. There is the common corn/maize starch, tapioca starch and yam starch. Notice most of these starch are either root based or cereal based, where these plant parts are used to store food for the plant. Gelatinisation occurs at different temperatures for different types of starch. As a general rule of thumb, root-based starches thicken at lower temperatures whereas cereal-based starches thicken at higher temperatures.
My preference is to use corn starch since it is cheap and readily available at all provision stores and supermarkets. I have a shallow enamel plate which I use for cooking starch paste. The plate is filled up evenly with starch to about half of its depth before water is added. As starch molecules will absorb water and clump together, it is important to keep stirring and to make sure no lumps are formed. Add enough water to form an even white mixture where you can see the starch powder suspended in the water.
The mixture is then placed over a small flame, again with constant stirring to ensure even heating throughout. The shallow flat plate is good as the heat is spread out over a larger surface area instead of a container with a tapering small bottom that concentrates heating at the bottom.
As you stir the heated mixture, you will encounter some resistance as the starch undergoes gelatinization, once a clear, gooey paste starts forming, off the fire but let the plate sit on the stove. Continue stirring as the rest of the mixture literally hardens as you stir.
The finished paste should have the consistency of sticky red bean paste and a translucent colour. Once done, cool the paste in a glue container. I normally fill up to half the depth of the container so that I can add in white glue or PVA up to one third depth of the glue container.
Stir the white glue/starch paste mixture well. The more liquid glue and the thicker paste should eventually form a mix that has the consistency of runny mayonnaise. The starch glue is the element that gives adherence between paper and boards without leaving unsightly glue stains from over application, whereas the white glue allows the mixture to dry more quickly.
The starch glue can be stored and used for about 5 to 7 days before mould starts to set on the surface of the mixture. I do not refrigerate my mixture to keep it longer as it will harden very quickly.
Hope the guide has been useful and keep trying!