Thursday, December 17, 2009

Plaster

A restful day but I have done a bit of casting. I've tried many way to obtain a smooth accurate relief including pouring plaster onto the painting surface, and cutting or carving images from solid blocks. This time I wanted to cut the shapes from foamboard and cast the plaster in the holes.

The foamboard positive, and cast plaster modelThe first results were not promising, there were large air bubbles, and keeping the surface flat and even proved problematical. The process was also very messy. It's naturally difficult to calculate the volume of plaster needed for pouring into a mould, so it overspilled.

I've just cast a second set, this time casting a mirror image, so the visible part will be the underside which is always better looking. The foamboard, though excellent to mark and cut, does bend and deform easily and it is difficult to demould the shapes. The card is sensitive to water so after just one cast the moulds are already showing signs of deterioration.

When making the Keyholes picture I concluded that the best system was so cast a solid block and carve, but that makes for a heavy picture, and it's not easy to guarantee a strong bond between the plaster and the wood panel surface. A picture should ideally withstand being dropped. I like to test every new process to destruction!

It's all a lot of work. At the moment I'm more obsessed with perfecting the process, less so with the artistry. I might not use many of the relief parts I'm casting because impressive as they look, they might not increase the verisimilitude of the painting. It's good to gain the practical knowledge and skills though.

6 comments :

Kathy said...

I like your experiments! Your creativity is balanced by working things out methodically. I've worked with plaster a few times, but not the way you are. One thing that might help you cast in foam core board is Vaseline (petroleum jelly) thinly smeared on the mold itself. This not only helps for unmolding the plaster, but also creates a vapor and liquid barrier between the mold and cast. Also, vibrating the wet plaster after it's been pored helps to release the air bubbles, assuming that the mixture is the correct viscosity and temperature. Good luck!

-Don said...

Mark, it sounds to me like you're having a lot of fun. I love to experiment. It's all part of the creative process. When you hit the right answer I'll bet it will be as rewarding an "Ah ha" moment as finishing a painting. Have fun! -Don

hwfarber said...

I wonder if a can of spackle (used by contractors) might work better than trying to pour plaster.

I've also played with spray cans of foam insulation--light weight and can be carved. It expands, though.

I'll watch with interest.

PAMO said...

I'll be interested in knowing how you attach the finished casting to the board. I like seeing the process. Thanks!

Mark Sheeky said...

Hmm, not heard of spackle. polyurethane foam is an interesting option, particularly for weight reasons. Perhaps polystyrene itself would work.

Hi PAMO! I've tested PVA, acrylic medium, acrylic gesso and molding paste to stick the plaster to the wood panel (which itself had already been sealed wit acrylic medium; Golden GAC100). I thought the molding paste would be a good choice because it's marble dust mixed with acrylic medium. They were all very strong, I had to use a chisel to get the plaster off! I chose acrylic gesso in the end because it seemed to dry quickly and had just the right consistency, with a bit of "grit" for extra grip.

PAMO said...

Thanks Mark for those follow up comments on attaching one thing to another. I appreciate the info!