advice needed on making a lightshade mold
Posted: 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/
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/