casting a section of a large sphere in polyester resin
I really like the lamp, though, as it matches several of the other lights that my wife and I have purchased, so I figured that I would try my hand at making a replacement shade!
The shade is shaped as follows: it is a section of a sphere 19.5" across (the diameter of the section, not the of the entire sphere), and 5" deep. A little math (x^2 + y^2 = 2yr) lets me know that the radius of the sphere that this shade is a section of is 12".
Here is my plan, and I would like to know if I am crazy or if anyone has a better suggestion. I am going to try to cast a replacement shade from polyester resin, 1/4" to 3/8" thick. I was going to make a 2-piece mould from plaster, and use my wood router suspended from a gimbal (on a 12" radius arm) to cut the outer mould. Similarly, I would suspend a plaster blank from the gimbal over my router (on a 11 5/8" radius arm) to cut the inner mould. Then I would simply coat the two halves with mould release and pour/pump the resin into the gap between the halves. I'll tint the resin with some dye to get it the amber colour that I want, then lightly sand to get a smoky translucent finish.
My only questions are: 1) will I be able to get this mould apart without destroying the resin casting, and 2) will polyester resin be strong enough for this kind of casting? Another option would be to make a wooden mould, but I thought that I might be able to chip away a plaster mould, whereas a wooden mould would be a lot harder to get off of a stuck casting!
PS, of course, if this works, I'll post an instructable of it!