I don't know about faster given that some of the schedules I have seen for doing telescope mirrors take DAYS to run, which I can understand on larger mirrors where I might be casting as much as 20-40 lbs of glass, and in some cases MORE !!!
All the glass I'm using is recycled, and the best I will ever be able to say is the front plate will always be one type of glass, and the foam glass will all be one type, but no assurance that the front and foam will be the same type. If you think that will cause problems getting the foam fused to the front, then I need to know now because I really have no idea what specific formulations of glass I am getting... all I know is that it is clear float glass, usually from discarded table tops. Some may be slightly colored but I avoid anything with heavy coloring. A bit of explanation about what can go wrong mixing glass formulations would really help about now
I think I need a better explanation of shock zone... but in case I got it.. you are saying that between cold and 500, that putting cold on hot will cause problems, but that once the front plate is past 500 that adding the foam mix is O.K. ?? If that is the case then why don't you like the idea of adding the foam mix at 1500 ?? Its not like I'm worried about the foam glass shattering before it heats up
If I'm going to add a thermocouple, it'll be in the bisque just under the surface at the peak of the mold.. I can't afford sacrificial probes
OK -- lets talk schedule for a minute.. my starting point schedule looks something like:
5h to 950 soak 0h
1h to 1250 soak 1h
2h to 1750 soak 1h
FAST to 1050 soak 8h (kiln open, vent running, close for soak)
30h to 950 soak 0h
15h to 850 soak 0h
20h to 90 off
Just over 3 days total run time, and I better start the run early in the morning so I'm into the annealing before I go to bed
![Smile :)](./images/smilies/icon_smile.gif)
This schedule is for a honeycomb backed mirror that is cast face up with plaster hexagons washed to the kiln shelf.. something of a different animal than what I'm doing, but at least its a starting point for the huge amounts of glass I'm casting compared to most art glass designs.
Based on what you just said, I'm probably going to stretch that 8h annealing soak to 12h, and add more time on the last 3 steps... maybe 35, 20, 25 to give the glass time to change temps more or less evenly. I'm also going to dump the peak temp down to around 1400-1500 since I'm not trying to get the glass in between a bunch of hex inserts spaced a fraction of an inch apart.. I just need whatever it takes to get the front plate to slump, and 1400-1500 is the right range for getting the foam glass to to gas up and fuse.
Still, with those changes I'm looking at a 4 day firing cycle, so you can imagine why I don't want to do it twice for a single mirror !! Even if the front plate were annealed and cooled before adding the foam mix, I still can't see it taking less than 2 days to slump and cool the front plate (slap me if I'm wrong here... I haven't looked into schedules for doing the front plate separately at all), which would push the total time up to 6 or 7 days... that is a LOT of time to run the kiln even if most of it is slow ramp down time which doesn't take that much power.
You mentioned that the foam glass is a good insulator, and its not going to be helping that the containment ring and "kiln shelf" are either castable refractory or fireplace insulating plate.. might as well think of both being very good insulating fire brick. I can get the insulation numbers from Corning for their foam glass products as a rough approximation.
And FINALLY... I'm not too worried about the top of the foam glass going solid on top, though it wouldn't bother me if it did. Since the glass is gonna spend at best an hour or two at fusing temps, that doesn't give much time for bubbles to migrate out, and I'm sure they will be replaced by bubbles from deeper in the mix.. I would actually expect the front plate to get thicker rather than the top of the foam glass glazing over solid.
There are no problems, only solutions.