Re: What types of chemicals can I add to gypsum to slow down its setting time.

To be honest I contacted US Gypsum, leading manufacturer of gypsum based products in the USA. Their technical staff advises using a commercially available product known generically as "plaster retarder" (mixture of hydrolyzed keratin and hydrated lime) . Or you can try "cream of tartar" (potassium hydrogen tartrate) which should be obtainable at your local grocery store.

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