Problems with Glass Slumping Molds

I'm trying to create different glass slumping molds for slumping, and recently they have not been working out so well. The mixture seems wrong but it is the same one I have always used. I just wondered if you know if flint can go off after a period of time, or if you know what might be at fault. I use the ratio of 1 pt water 1 lb plaster to 1 lb flint or quartz. Recently they have not been combining well and seem too wet. Any advice you could give would be much appreciated.

Milly's suggestions: Hi Judith, thanks for your question. Moulds - argh! They can drive you mad... Flint can't go off, no - so that's one potential problem out the way.
With regard to the ratios - did you have a look at the recipe on my Mold Making Instructions page? I find volume easier than weight, so I have three large yoghurt cartons and measure 1 flint, 1 grog, 4 plaster and 2.5 - 3 water. A bit unscientific I know, but you can always add a bit more plaster if it looks too wet. It is a moveable feast - sometimes the same recipe you've been using forever suddenly need adjusting for no apparent reason.
It looks like you're using 1:1 ratio, with no strengthening grog? The grog - fired clay broken up into tiny pieces or powder - strengthens the glass slumping moulds, whereas the flint allows them to resist high temperatures. You can buy grog from pottery suppliers, it may help.
One last thing; I'm assuming that you're putting the plaster in last? That's crucial, and should be sprinkled (sieved ideally, but careful of the dust) to create little peaks poking out above the water before you start mixing. If you're not getting peaks of plaster, there's not enough. It's difficult to explain without seeing it, but I hope this is helpful.

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