Half an hour before the plasterer came, I took off the light switch and the plaster round the edge fell away. No problem, bit of thinned PVA, out with the old bag (nearly a year) of Wickes one coat plaster and sorted - it sets in 2 minutes ! But I did wonder - why exactly does old gypsum-based plaster set so fast ? Has the plaster changed or has a retarder degraded letting it set at its natural pace ?
Simon.
I think the gypsum has absorbed moisture from the air. This forms hydrated crystals like the ones that form when the plaster sets.
When you add the water to the powder to make the plaster properly, the hydrated crystals act as seeds for the setting process and it goes much faster. You see the same effect if you mix fresh plaster in a bucket with traces of set plaster in it - the mix will set very quickly.
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