Why does sodium sulfate initially harden instead of dissolving? I was recently drying a product by passing it through a bed of sodium sulfate in a fritted glass Buchner funnel. Once I had finished up working my reaction, in an ill-advised attempt to clean up, I decided to wash the salt out with water before proceeding with acetone. Upon adding water, the salt starting foaming, and became hard as a rock and adhered to the fritted glass. I decided to leave it sitting in the water, and after a while it did finally turn gel-like and was able to be washed out.

My question is, why in the presence of so much water, why did the salt remain as the hydrated salt instead of dissolving, and why did it take so long to dissolve after it formed the large crystal? Also any idea what the bubbling was?
Sodium sulfate forms various hydrates including a decahydrate. The dissolution is exothermic which saturates the solution. When the warm solution reaches the relatively cold frit the supersaturated solutions precipitates. In the glass frit the pores would clog and it would take time to dissolve the sodium sulfate out.