A technical dictionary of printmaking, André Béguin.
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Tympan
The tympan is used in lithography and covers the paper during printing. Its function is to distribute the pressure evenly over the lithographic stone, during printing. Without the tympan the scraper bar would damage the paper during the printing process. Normally the tympan is made of this flexible material like plastic or metal. Some people recommand plastic sheets of high impact glass epoxy (Panelyte 161 type Gee., .031 gauge. *Tamarind book of lithography). Other prefer a tympan that is made of "Lexan" (acrylate) because it is transparant and does not break so easily. The transparancy allows you to see what happens with the paper during printing.
In the old days a sort of pressboard was used, only because there were no other flexible materials available. It was hard to clean and deteriorated rapidly. A plastic sheet of grey PVC of 1mm this does also perform very well. The only thing a tympan must do is transfer the pressure to the paper, while it slides under the scraper bar. So it should not raise to much resistance while sliding (use simple ball bearing grease to lubrificate it) and it must be able to resist the pressure without changing form, curl (like metal sheet does), or desintegrate.
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