Totally naive about a possible process.

[Wendell Brown]

Nice thought points all. Appreciate the info. Fiber blanket walls and roof. Hadn't considered that tack. Any suggestions of brands? I suppose plate glass will need heating to around 1500-1700F for the process. Annealing seems like a controllable process when one doesn't ramp down too fast. I can see I need to study this better but you all help a lot in that process. Now if I can figure out why one sweeps through the glass vitrification stage ASAP.

[Bert Weiss]

You said,"I don't know a single advantage to a brick wall. Well . . . several of us do, long slow anneals. That heat that has been put into those brick walls is released slowly and assists in the annealing cycle. I tried to give you a hint, "Monolithic float versus multiple piece art glass" but you never get them . . .

For the particular purpose described, a thick casting, I would go with a brick kiln.

Sorry, I strongly disagree. First of all, monolithic float glass and multiple piece art glass anneal much the same, as long as thickness profiles are the same. They behave quite differently on the heatup cycle.

Controllers are for establishing firing schedules. Element placement is another important factor. For a thick casting, you want elements in the floor and roof for slabs, or all 6 sides for an investment casting that has dimensions different than a slab. This will establish the evenivity you need quicker. Annealing actually takes place in 15 seconds. The trick is getting the glass to within 5ºC for those seconds. The more even the heat is inside your kiln, the quicker the job gets done. Unfortunately for us, we don't generally know when this happens.

For the kind of fusing most of us do, brick is second rate material for walls. The companies that make these are stuck in the WW2 era, and never changed their design as technologies developed and grew. Depending on the thickness of a brick floor, top elements are sufficient up to around an inch and a half or up to 2" for a 5" floor. You must be able to put more heat in from the roof elements than you are losing through the floor. Thicker than that, bottom and top elements are required, and will speed anneal time considerably. For stacked glass fusing, side elements are a horrid idea, unless they are well blow the glass edge level. They will make the edges seal quicker ending the possibility of a bubble squeeze.

The only application I know of that benefits from brick walls would be a glass blowers annealer that is often opened and closed. The bricks will hold on to the heat and not create large swings. On the other hand, I have worked with fiber blanket lined annealers that work fine.

[Morganica]

Bert, the kind of fusing that most of us do really isn't applicable here (nor is it applicable in a significant portion of my work). I would agree that if you're looking to precisely control the temperature all the way through the cycle, fiber has an advantage precisely because it doesn't retain heat well. You can spill or add heat pretty much as needed.

Doesn't mean there aren't reverse situations--there are. And there are absolutely ways to take advantage of that extra insulation that compensates for lack of zone control, bottom elements, or other elements in the wrong place(s).

I can (and will) figure out exactly when, where and how my kiln is affecting the glass (and am actually working with one of our thermal analysis engineers at work--he's been trying to talk me into using our software for months)...but I suspect I'll discover that my overinsulated firebrick kiln has been doing much of the work for me.

[Morganica]

Nice thought points all. Appreciate the info. Fiber blanket walls and roof. Hadn't considered that tack. Any suggestions of brands? I suppose plate glass will need heating to around 1500-1700F for the process. Annealing seems like a controllable process when one doesn't ramp down too fast. I can see I need to study this better but you all help a lot in that process. Now if I can figure out why one sweeps through the glass vitrification stage ASAP.

Wendell, if you review crystallization, you'll have most of the info you need on DEvitrification, i.e., "unglassifying." It's literally about an amorphous solid becoming a crystalline solid (in spots).

Devit can occur in many situations, but it's more likely to occur within what's called the "devit zone," which for soda-lime fusing glass is typically 1350-1400F or thereabouts. The glass is soft and moving, able to sinter, and if left too long can start forming crystals around nucleation sites on the surface (dust/dirt/pits/abrasions/etc.). You rapidly drop temp through that devit zone (actually from process temps down to anneal soak) so the glass doesn't have enough time to form crystals.

Devit starts as a cloud or whitish surface scum that doesn't scrub off. It's actually tiny crystals that will continue to grow with additional heat/firing cycles, eventually making the glass look wrinkled. If it keeps on, the glass will fully crystallize and become a crumbly white mess.

You get rid of devit by grinding it off, covering it up or by sending it back to the furnace for remelting (at temps far higher than fusing temps).

Most casting and fusing glasses are formulated to resist devit. Soda-lime seems more prone to it than lead crystal, opaque glass more than transparent, smaller particles of glass (frit) more than large, solid pieces, and glass in a plaster casting mold more likely than glass on a kiln shelf. Some brands of float glass are notorious for devit--I use devit to create lines and movement in some of my float glass castings:

The whitish lines throughout the piece are devit. The back of the piece is dramatically devitrified:

A light coat of devit really isn't a problem if you're grinding the glass to its final shape. It's a surface artifact. You just want to avoid an extended crystallation that would penetrate past the final surface you're trying to achieve.

[Bert Weiss]

Bert, the kind of fusing that most of us do really isn't applicable here (nor is it applicable in a significant portion of my work). I would agree that if you're looking to precisely control the temperature all the way through the cycle, fiber has an advantage precisely because it doesn't retain heat well. You can spill or add heat pretty much as needed.

Doesn't mean there aren't reverse situations--there are. And there are absolutely ways to take advantage of that extra insulation that compensates for lack of zone control, bottom elements, or other elements in the wrong place(s).

I can (and will) figure out exactly when, where and how my kiln is affecting the glass (and am actually working with one of our thermal analysis engineers at work--he's been trying to talk me into using our software for months)...but I suspect I'll discover that my overinsulated firebrick kiln has been doing much of the work for me.

I agree with you Cynthia with a small exception. I was once told that 1" of fiber board is as insulating as 9" if insulating fire brick. I'm skeptical the fiber is that much better, but it is better.

I'm not a telescope guy, but don't they use borosilicate? I also haven't worked with boro, but it is surface hard, has a high softening point, and is prone to devit. I agree that devit on the surface is a non issue as you will be grinding and polishing it off.

As to my opinion about fiber walls, it has everything to do with evenivity. It is my opinion that many people load their kilns with glass too close to the kiln walls and then have breakage problems down the road, either on subsequent firings or just sitting in a room. These problems are not easy to diagnose. Part of the problem comes from the widely held myth that you can not anneal soak too long. You can if there is uneven temperature that your glass "sees".

I am usually astounded at how commercial companies design kilns. From what I see, they tend to design based on 2 factors. One is that they make kilns the way they always have since WW2. The other is that they cut corners to keep the cost down, in ways that are not cost effective in the life of the kiln. There are some exceptions, like the Skutt Clamshell or the Paragon Pearl series. Every large bell I have seen made by a company has a rolling floor. What glass artist wants to set up a delicate stack of glass, maybe on a delicate sand mold, and then roll it? Blanket lined kilns are less expensive to build, but the cost of lining with fiber board will make the unit last quite a bit longer. Control systems are mostly really cut corner. Mechanical relays should never be used (with the exception as part of a hitemp limit system). There should be redundant hitemp limit controls. Today's controllers can do amazing things with solid state relays that cost about the same as the mechanical ones, like cycling them 120 times a second or faster.

BTW, I have been talking with the folks at Arrow Springs and Tekniv about their Tekdaq based controller. If you have several kilns to control, this is a really neat controller with more capabilities than anything I have ever heard of, including the capability of monitoring and changing it from your phone or computer, while in another place. It will sense a fraction of a degree change in temperature, and change power output by a fraction of a percent. It is industrial grade and expensive for one kiln, but if you have several, it becomes a bargain.

[Morganica]

BTW, I have been talking with the folks at Arrow Springs and Tekniv about their Tekdaq based controller. If you have several kilns to control, this is a really neat controller with more capabilities than anything I have ever heard of, including the capability of monitoring and changing it from your phone or computer, while in another place. It will sense a fraction of a degree change in temperature, and change power output by a fraction of a percent. It is industrial grade and expensive for one kiln, but if you have several, it becomes a bargain.

The Digitry DB5 does that.

[Bert Weiss]

BTW, I have been talking with the folks at Arrow Springs and Tekniv about their Tekdaq based controller. If you have several kilns to control, this is a really neat controller with more capabilities than anything I have ever heard of, including the capability of monitoring and changing it from your phone or computer, while in another place. It will sense a fraction of a degree change in temperature, and change power output by a fraction of a percent. It is industrial grade and expensive for one kiln, but if you have several, it becomes a bargain.

The Digitry DB5 does that.

I know. But, there are several advantages to the Arrow Springs model, like 48 inputs so you can throw in extra thermocouples to watch temperatures beyond the control ones. They don't require thermocouple amplifiers, and there are 24 outputs. The 2 are going to be relatively close in cost. The Arrow springs model comes with it's own computer included. I'm not sure if you need a monitor or not.

[Brock]

Wow, sounds great. You should probably get one. It would make your highly technical deformed float over crumpled fiber paper easier to achieve. You probably wouldn't have them break so often.

[Haydo]

Thanks Cynthia, for posting those awesome images of that cast face again, I had meant to praise the work the first time I saw them sometime last year. peace- haydo


PS. and peace to warring parties, maybe a jelly pool fight is in order. Couldn't watch if each contestant started licking each other though!!!

[Ron Behrens]

Hello
I am likewise in the learning stages yet of making meniscus mirrors. My plans are to slump flat glass over a precision mold
In my case I am making the mold out of plaster silica with chicken wire inside and casting it in a mirror blank that has been coarse ground. I have tried it with scrap glass and seems to work. I am currently working with an 18 in and plan to make a 32. Also Mel Bartels website has information
Ron Behrens