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Mix all of the ingredients together in a pot in the amounts above, and stir until agar and glycerol dissolve in the water.
Put the pot on the stove and heat the mixture to 95o C or to just below boiling. Keep stirring the whole time. When it reaches 95o (or begins to froth), remove from the heat. Keep stirring and skim off any of the froth from the top with a spoon. Any froth left on the surface will cause air bubbles in your plastic sheet.
Pour liquid onto a flat surface or mold. A silicon sheet works well with a frame (you can cut this from scrap wood), or alternatively, lay a silicon sheet on a baking tray with edges. Let dry for 1-2 days, depending on temperature and humidity levels.