Air Bubbles in Plaster

Q: When I cast plaster into a rubber mold, how can I prevent bubbles from sticking to the mould and making holes in the casting?

A: Using a wetting agent will help, but it will only solve your problem partially. Bubbles tend to migrate toward the top of the mold, especially if you use vibration to dislodge them, but will accumulate in upward-facing concavities. My favorite wetting agent is tincture of green soap (available from chemical or pharmacy supply houses) diluted with alcohol to a water-thin viscosity. It is best sprayed on and allowed to dry before pouring the plaster. Various proprietary compounds (“debubblizers”) are also available from sculptors’ and jewelers’ supply houses for this purpose.

But the best solution I’ve found for air bubbles is to boil the plaster at room temperature, by placing a half-full container of the mixed plaster in a vacuum bell jar and reducing the atmospheric pressure to 29 inches of mercury. This will greatly reduce the air suspended in the plaster mix. If you pour the plaster into the mold slowly, without turbulence, you will have a lot less problem with air bubbles. But if you want to eliminate them entirely, you need to place the (not entirely) filled mold in the bell jar and vacuum it again. If you intend to do this, don’t use a soap-based wetting agent, as it will tend to foam in the vacuum. You might try one of the vacuum-friendly surfactants such as “Vacufilm”, which are used with jewelry investment, for this application. This solution is borrowed from jewelry-making technology, which in turn adapted dental casting techniques, and is only useful for sculpture small enough to fit in one’s vacuum bell-jar. Larger pieces must rely on wetting agents and—for large flat pieces—splash-coating techniques.

I have also heard of systems which use pressure instead of vacuum to accomplish the same result, but have never tried this, although the results are supposed to be similar.

by Andrew Werby