Firing schedule/problems

[Mark Selleck]

I'm the crazy person trying to make lampshades out of wine bottles. The success of my second attempt had me thinking(?) that I had it figured out (successful 2nd attempt @ http://www.glassimpressions.net/bottles.htm ). My third attempt ended up like the first failure: several pieces, apparent thermal shock on the ramp up. There WERE some differences this firing: different bottles(thicker glass), and the position of the form/wok(upside down) was higher in the kiln, to get more heat around the bottom of the shade during the final stages of the slump. The bottom opening of the wok is about 2" off of the bottom of the kiln, resting on 3 fire bricks.

The elements closest to the bottle "disc" edges (disc 28.5" diam., kiln 30" x 60") were baffled with 110J. The disc was centered on the top(bottom) of the wok, the only contact being the "hub" of the disc, where the bottle "openings" touch each other forming a ring in the center of the disc, with about a 2.5" opening in the very center.

Possible reasons for failure: Raising the wok in the kiln brings the highest point ABOVE the position of the temp sensor, which means the initial temperature (50/hr ramp up)is a little higher than with my original positioning. The glass in the disc is very irregular in thickness across the disc, though radially symetrical. Since the ONLY contact, initially, with the wok, is in the CENTER(one of the thicker glass areas), is it possible the wok is heating up and transmitting too much heat to that area? Would a couple of thicknesses of 110J between the glass and the wok help in this circumstance? Or, should I just lower the wok back to it's original position, and drop the ramp up to 25/hr? Any and all information/theories would be appreciated for consideration.

Thanks,[/b][/u]

[Kevin Midgley]

I think because of all the variables when working with bottles, the slower ramping you suggest is important. In the lampshade you made, there are some folds which indicate even thicker glass. Because your mold was higher in the kiln you were closer to the elements and there was less distance to even out any heating irregularities that would normally go unnoticed. I do not use fibre stuff in my kilns to protect an area from heating for I feel it is just better to go slower heating and cooling. The obvious might also have been missed in this case, that all the bottles although looking the same, were not actually the same, some having been made on different days/different glass batches etc. I have made these large bottle bowls and found slow is the only way to make them. Kevin in Tofino

[Mark Selleck]

Kevin, thanks for your input. You are correct about the folds in the completed shade, however those don't appear until after the slump is complete, so that added thickness is "after the fact". This third attempt WAS with a thicker bottle, thus a thicker overall blank. My bottles are all purchased new, from a single manufacturer. That said, I know you're correct that it is possible for different batches of bottles, with different COEs, to get mixed. No way to control that possibility, unless I get to the point where I want to buy direct from the factory, and can then specify that all the bottles come from one batch.

In doing the final slump I am using only the side elements in my kiln, as they are the only elements that are computer-controlled. Given that, do you think there is any reason to drop the mold back to it's original position in the kiln? I think I will try the slower ramp, as you suggest, though adding another 22 hours to the ramp up isn't a pleasant thought!

Thanks again for your help.

[Kevin Midgley]

Lowering the mold would allow more air circulation around the top of the mold and into the center. You would not have to worry about the height of the mold if you had heating on the top of the kiln. Want a cheap Canadian made controller? I can put you in touch with someone who buys perfect fires 10 at a time. I will have to check my schedule for bottles but I don't think it is that long. I don't particularly like steel molds because of the lack of thermal mass when doing fusing projects like yours. Kevin in Tofino

[Mark Selleck]

Kevin, thanks for your help. I'd be interested in the contact re the controller. I'm thinking of changing the top element layout of one of my kilns for more even heat distribution, and adding a controller would be another definite improvement.

If you wouldn't mind sharing your bottle schedule, I'd be interested in that, also. Up to this point I've been ramping up at 50/hr to 1100, hold an hour, 50/hr to 1350, hold 90 min, AFAP to 1000, hold 3 hrs, 25/hr to 800, 50/hr to 600, 100/hr to 200, cool naturally to room temp. I was being very conservative, I thought, after my first failure, and the finished shade checked out stress-free with a polarizer. I'd like to go faster, obviously, especially if I slow the ramp up, but annealing is just as important, and I don't have enough experience to tell me if I can speed up the down cycle.

Thanks again.