Mold making pointers

COMPANION TECHNICAL GUIDE: Casting with Plaster: Techniques check list
(also see Rubber Mold making post)

Making a plaster positive from a plaster or wood mold:

¸ Seal porous surfaces with shellac (thinned with alcohol 50/50) 3 COATS (dry in between)
¸ Be sure mold is well supported
¸ Use clay to dam edges of mold
¸ Use release agent on your mold so plaster does not stick to it
¸ For large molds, keep plaster light by reinforcing it with burlap or cheesecloth
¸ Don’t wait too long (a day is too long) between batches – plaster will have a seam or break
¸ De-mold after plaster has gone through its exothermic reaction (after it has gotten hot and then cools down)
¸ CLEAN UP - GOOD HOUSEKEEPING!!

Making plaster positive from a plaster mold:

¸ Seal original object to be cast if needed with shellac (thinned with alcohol 50/50) (dry in between coats – 3 coats)

¸ determine PARTING LINES of your object. Determine the number of pieces of mold to avoid undercuts.

¸ create dam wall in clay along the parting line. Clay wall will support lip of plaster
¸ put keys into clay

¸ apply RELEASE AGENT to object’s area to be cast

¸ cover object and dam wall lip with a SPLASH COAT. Blow on splash coat to push it against object and eliminate air bubbles.

¸ Wait for plaster to thicken slightly. Apply thickened plaster to top of face coat. shape into smooth dome.

¸ After exothermic reaction takes place, carefully remove clay from object and plaster. Build new dam wall if needed DO NOT REMOVE POSITIVE.

¸ Be sure to use MURPHY’S OIL SOAP AS A RELEASE TO SEPARATE PLASTER FROM NEW PLASTER

¸ When mold is finished, coat interior with Murphy’s oil soap to release plaster cast.