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Hi all, I recently learned to slush cast in latex. After about a week, one of my plaster molds grew mold (as in fungus) on the outside. It's definitely mold (a yellow coating with green spots), not mineral deposits. Has anyone else seen this before?

Some info: It's plaster of Paris. I did use an old milk carton to mix the plaster---maybe I didn't clean it well enough. I've also been using corn starch instead of talcum powder for de-molding, so I wonder if that is feeding the mold.

In the meantime, I started mixing plaster in a new container from the hardware store. I'll post back in a week or so and update whether that fixed it.

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If kept in a dry environment and not encapsulated this shouldn't be a problem. Sounds like the mold has be damp for an extended amount of time and not have had the chance to dry out after use. Keep your molds in a heated room like your boiler room or the like. Spurs are all around us all the time, and given the right circumstances will sprout. While working in a ceramic workshop we kept the molds on shelf's high up and with the ovens running frequently it never showed a problem. To me it sounds like a general humidity problem, that you should deal with asap, unless the issue is that you wrapped the mold in plastic. If you're working in a little unheated shed, a little solar driven ventilator can work wonders, keeping it dry. For your current mold I would dry it out in a heated room for a couple of weeks, and then when sure that it's dry, put it in your kitchen oven some were between 120-160 degree Celsius for two hours, to kill the fungus and spurs. Going higher might have an impact on the constitution of the cured plaster, making it brittle. And if not completely dried out it may crack. You could use the oven all the way starting well below boiling point for some hours and then gently raise the temperature. The time depends on the mass. Just make sure that it's heated good all the way through.

Hans, thanks for the advice. I think you're right. Even after I switched mixing container, a newer plaster mold has also gone bad.

More info: Yes, it has been very humid where I live recently (although I did not wrap the molds in plastic). I've also used these molds to make a large number of duplicates, repeatedly demolding and repouring the latex without much time in-between.

I'll try baking them as you suggest.


Hans Jacob Wagner said:

If kept in a dry environment and not encapsulated this shouldn't be a problem. Sounds like the mold has be damp for an extended amount of time and not have had the chance to dry out after use. Keep your molds in a heated room like your boiler room or the like. Spurs are all around us all the time, and given the right circumstances will sprout. While working in a ceramic workshop we kept the molds on shelf's high up and with the ovens running frequently it never showed a problem. To me it sounds like a general humidity problem, that you should deal with asap, unless the issue is that you wrapped the mold in plastic. If you're working in a little unheated shed, a little solar driven ventilator can work wonders, keeping it dry. For your current mold I would dry it out in a heated room for a couple of weeks, and then when sure that it's dry, put it in your kitchen oven some were between 120-160 degree Celsius for two hours, to kill the fungus and spurs. Going higher might have an impact on the constitution of the cured plaster, making it brittle. And if not completely dried out it may crack. You could use the oven all the way starting well below boiling point for some hours and then gently raise the temperature. The time depends on the mass. Just make sure that it's heated good all the way through.

You could also try add some anti fungus product - like the ones you use for making marmelade, to the plaster while mixing. But without sugar!! Don't know what that would do to the plaster, but nothing good I gather. Best of luck I hope your mold survives the treatment!!

And don't forget to dry your mould out gently in the oven once it has set. They contain quite a lot of water - all that you mixed in!

I think mould spores are everywhere, so if the conditions are right the fungus will grow.  So drying the plaster mould is probably the best approach.  Liquid latex works best in a dry mould, the dry plaster absorbs moisture from the latex. That's what creates a thickened layer that stays on the mould surface when you pour the excess off, which dries to form the skin, so drying the plaster is a good idea anyway.

 There are anti-mould additives for paint you could try, maybe brush or spray a little on plaster that has already gone mouldy. (I haven't actually used it on plaster, so I can't be sure it is a good idea.) 

...And I imagine, like all living organisms, the fungus wil prefer the nutrition of corn starch to talcum powder (which is basically ground-up rock and perfume)

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