Metal Casting Zone

Sometime ago I asked about and was given sound advice about Lost Foam Casting for that I thank those Posters.

I have moved forward and started Lost Foam Casting with moderate success. One thing I have noticed is that on flat surfaces there are little beads/balls of aluminium.
The sizes are between 0.5 and 1mm in diameter. As per the photograph.

My process has been to make the mold and coat with a plaster skin and then cast, not always does it happen but I have an application where I must have a very flat surface
and I cannot machine it because it is the inside of a Thermal Camera housing.

I was wondering if this was caused by moisture in the plaster skin?

It couild have been an air bubble that formed on the foam or a water droplet that made a void in the plaster. I would vote for the air bubble. You could thin down some plaster and paint it on and then coat with thicker stuff. I've done this on wax molds. I don't know if it works yet, I haven't had a good enough pour to tell.

Had a similar problem and found vibrating the coated form (in my case tapping the base rapidly) released bubbles. Most recently however been coating the pattern with a dilute solution of dish detergent and water to lower surface tension as plaster contacts pattern)( still tap also,old habits are hard to break). Seems to work most of the time.
Hope this helps.
Blindpig