advice needed on making a lightshade mold

Richard Dubois · Post by **Richard Dubois** » Sat Aug 30, 2003 8:26 pm

I am attempting to replace the ceiling lightshades on new fixtures we got for our kitchen remodel. The commercial shades are 12" across and 2.5" deep. They have a 0.5" flat rim that is captured by spring-loaded holders (look like C's). I need to make 4 of these shades.

I made a mold using Type B mold mix (I think it was Type B) - it was 25 lbs of mix poured over the commercial shade into a plastic stepping stone mold (with a cardboard dam to allow a taller pour).

After a week's drying, I fired the mold to 300F for an hour and then to 1500F for an hour. Then kiln wash. I slumped one test shade in it and found many hairline cracks radiating out of the center. Hoping they were just surface cracks, I was going to paint a slurry of new mix to fill them in, but the mold disintegrated when I carried it away from the kiln.

In addition, the slumped shade was cut to the full width, including the 1/2" rim, but the glass just folded up into the mold and there was no flat rim.

So, two questions - how can I make a durable enough mold to make my 4 shades; and how do I get the 1/2" rim? I'm guessing that some amount of oversize will result in a flat rim. It could be trial and error to find the minimum oversize to get the closest rim and then grind or saw out the rest.

Here is a pictorial history of the attempts so far:

http://www.stanford.edu/~dubois/staine ... odel2003/

Melodie · Post by **Melodie** » Mon Sep 01, 2003 4:16 am

Iâ€™ve never used that type of plaster so I really can not comment on it. I would think that if a commercial slumper is not available in that size then it might be best to contact someone in your area that does pottery that can throw a bowl that size (not sure about a flat rim but want to say that a good potter could achieve one) or someone that does slip casting. A mold that size shouldnâ€™t be very expensive, especially done locally.

If you have access to Lundstromâ€™s first book he shows placing a second ring mold on top of the glass to keep the rim from slipping in. This could possibly be weighted down even further if needed. Canâ€™t guarantee results, Iâ€™ve never tried it but thought it might be worth a thought.

Melodie

Laurie Young · Post by **Laurie Young** » Mon Sep 01, 2003 4:57 am

A good potter should be able to throw this shape, maybe lay out glass a bit larger than you need and then cold work to fit, they are fairly shallow so it shouldnt slip into the middle, go up slow and slump at a low temp. Plaster molds dont tend to last very long, but having said that I have used ceramic slip molds for multiple firings.