Air Flow and Anneal

[twin vision glass]

Hello! I have been thinking a great deal about glass Artists firing on the bottom of their kiln floor in recent months. My feeling is that you would not get a good anneal if the piece was fired on a insulated fiber board kiln shelf on the bottom of the kiln. Fiber board is an insulator and the air temperature above the glass would not be anything like the temperature below the glass and especially when in contact with fiber board and kiln bricks below it. I fire my pieces in the centre of the kiln on Dyson shelves with air flow through the kiln shelf, up off the floor on kiln feet. My feeling is the glass is getting a better chance to anneal properly and evenly. The discussion I had was that the other artists felt it did not matter, but I am feeling quite differently. It was a really good discussion, and we both would love to hear other artists and technicians input on this question.
Thank you so much for the time and discussion that I hope will follow so I can answer this question to other artists who ask the same question. Leslie

[Bert Weiss]

Leslie

My kiln fires glass up to 40" x 84" with 4" surrounding the glass, all around. My floor is 5" of brick that is covered by many layers of fiber blanket and boards. I fire directly on the floor for everything but a drop slump. Sometimes I use a sand bed on the floor. I have a set of mullite shelves that have a ribbed bottom, that allows limited airflow beneath the shelf, that are placed directly on the blanket.

I have successfully annealed glass as thick as 36mm this way. I have not tried to work any thicker than that. There is a point where you will lose more heat through the floor than you can replace with the elements. Once you hit this threshold, annealing is impossible. Kilns with 2.5" brick floors will hit this threshold quite a bit sooner than on my floor.

I routinely anneal 10mm glass, and then have it tempered. In the tempering furnace, the glass is heated from room temperature to 600ºC in 5 minutes without any breakage.

Clear float glass does not "see" radiant heat from the elements. The heat goes through the glass and heats the blanket (or whatever is there) first, and then the heat is conducted through the glass from beneath. The surface of the blanket gets hot, but that heat is not transferred quickly through the blanket, as it is an insulator. Colored glass does "see" the radiant heat. I notice no difference in annealing both kinds of glass. What I did notice is that when I place 2 sheets of clear 10mm glass on a sand mold, with a depression in it, the bottom glass slumps before the top glass and before the edges seal, leaving a small bubble. If the top glass heated first, the bubble would not be created. I have only roof elements.

[twin vision glass]

Excellent discussion and one that my friend had the same feeling. BUT when I was attempting to fire one inch thick large cast pieces with multi layer Opal and Transparent colour bar pieces in my flat kiln with no bottom elements, I was having problems. Ted Sawyer suggested I get bottom elements and larger kiln and have the heat circulating , and now I am having 100 percent success (unless the power outage causes me grief. ) but I am glad to hear your words Bert and will pass them along to my friend as well. It has always caused me great pause to explain to others why I feel the other is better. Do you think Float Glass is more forgiving. I am using so many bits and pieces and slices , that perhaps it is harder to get a well rounded anneal on the kiln floor . It just seems to me to make sense that there is real different things going on with Fiber Board insulating and then holding the heat longer (oh , or is it throwing the heat) Bricks keep the heat. Menopausal moment . I was sooooo interested in Ted's talk about annealing and placement of work at BeCon a few years back that I really want to understand it.
Leslie

[Bert Weiss]

I would love to have a set of floor elements that have room for fire brick supports for a shelf system. WIth this setup you could do most any thickness, as long as you wait for the heat to reach the center. Heat from bottom and top would speed the anneal time considerably, as the heat would equalize much faster.

I can't speak to your problems. There are so many variables. The dams could be messing around with heat distribution. Your thermocouple could be way off, etc.

[twin vision glass]

After I was having all those problems I got a system set up like Ted Suggested of digital thermocouples set up in 4 places else where in the kiln and it is really great even heating now. My friend felt her set up is good, but I worry for her when she begins to go thicker . So thank you Bert for sharing some of your expertise. I am lucky now to have my main therocouple right in the center of the kiln just above the glass. 4 inchs on all sides, Dyson Shelves with the air flow and the dams are dyson tooo with air flow so that there is only 1/4 of an inch of mulite around the whole piece of 1 inch or thicker glass at the end of the day. So hopefully sharing this with others will help them to feel good about their firing set-ups as well. I still thank Ted Sawyer for his brilliant mind and helpful hints also.
Sincerely, Leslie

[Dick]

Hi Les! (and Bert). This is a good discussion, and both sides have merit. Of course you can fire either way if your schedules are correct. To me it only makes sense to have airflow. I was in a lecture by Richard Whitely who has done extensive research, even to having up to 4 pyrometers. He found that the temperature difference is vast from the floor, to the top, to the center of the shelf and the center of the glass. It only seems logical that we would do whatever we can to minimize that. Some of that is harder to do, as adding elements, but it seems a fairly easy compromise to raise the shelf up. That being said, you may have a project that due to the size or other parameters won't work on your elevated shelf. In that case you might have to be on the floor, but I would be way more conservative. It would take much longer to equalize the temperature differences between the floor, glass and air. Then there are people using thinfire right on the bricks which has the opposite problem that fiber has. It will take forever to lose the heat, hence your glass will be much hotter than the kiln states. It goes back to knowing your kiln, knowing the science of fusing, and making accomodations accordingly. Just like there is no #1 fusing schedule.

Dick

[Morganica]

I've fired on the floor (in and out of molds), on raised shelves, on sand, on sand on a shelf (that was a mistake, the shelf cracked), and with an extended shelf that literally jammed into two sides of the kiln so I could do an extra-long window.

But I wouldn't go to the trouble unless there was a pretty good reason. The inside of my kiln, an old Skutt GM1414, can vary nearly 50 degrees on a very fast upramp, one reason I tend to fire slower than a lot of people here. I'd rather not compound the problem by introducing new variables.

In most cases I made the different configuation work, withk extra time spent working it out logically and maybe experimenting to figure out how the change in firing position was going to affect things. (And found that firing fused work directly on the kiln floor is an excellent way to thermal shock the glass on the way up, somewhere between 500 and 700F. I suspect it's because the underside of the glass against the shelf is a lot colder than the top surface, but I haven't verified that.)

As Dick said, that might be due to the height or length of the piece, or to a disinclination to stick a wet mold on a perfectly good kilnshelf, the need to grab 3-4 more inches for an oversized piece, or maybe to baffle some particularly sensitive part of a mold.

[twin vision glass]

I am wondering also if you do fire on the bottom of your kiln and your thermocouple is up in the middle or at the end or some other place is the temp really reading right for the anneal. I just cannot think it could be. But also like everyone else, you must know your kiln , and do tests and be prepared to make some changes. GREAT discussion as I have said. Thankyou. Les

[Bert Weiss]

I am wondering also if you do fire on the bottom of your kiln and your thermocouple is up in the middle or at the end or some other place is the temp really reading right for the anneal. I just cannot think it could be. But also like everyone else, you must know your kiln , and do tests and be prepared to make some changes. GREAT discussion as I have said. Thankyou. Les

Thermocouples read air temperature at the point of the end of the thermocouple. My observation is that anneal schedules are universally applicable, as opposed to heatup cycles that are subject to local conditions. This is because the glass goes through a rampdown, at a particular rate. This is not precise and the same schedules seem to work. (the place where this is not true is when there is a temperature differential inside the kiln, and the program is too slow. This can introduce the temperature difference in to the glass and make it impossible to anneal.)

I have worked with a kiln that has the thermocouple tip sitting right in one corner of the kiln, and also, 2" from a quartz tube with an element in it. As far as I can tell this is controlling the kiln based on the element temperatures. I have programmed this kiln much the same as I would control mine, where the thermocouples are set between elements and glass. It worked. In that kiln, I have worked both on the floor and on raised shelves for a drop slump.

[twin vision glass]

I guess my first question was not perhaps as clear as it should have been. I am speaking more of long flat panels that span the greater length of the kiln . It does create alot of difference in temp. if firing on the bottom of the kiln when perhaps even the bricks at each bottom end of the kiln cool faster than those of the middle bottom creating I would imagine all sorts of things within the glass and at each end. But at least I am feeling quite confident with raised full sized shelf with air flow , lower elements , kiln shelf in centre, lots of room around outside so piece is not full to outer corners (which can create their own problems ) and I always was thinking that the dams would help to not let the glass cool around the outer edges if no elements are there. Lots to think about. If anyone is questioning there anneal temp, then get the hand held digital Thermocouple readers and set them in all the opens you can find to check your own kiln and see exactly what is happening. I would be so curious to see a thermocouple in the fiberboard under the glass and then just above in center and at ends. BUT WHO has that kind of money perhaps. Well Bullseye did this though and all the technical talk is on their site from the BeCon Discussion. Daniel Clayman was on the panel tooo if I do remember. It was such a great talk. Who else heard it. Incredible and have thought of it often. Les

[Bert Weiss]

Before I designed and built my big kiln, I asked Henry Halem if I could anneal glass on the floor of a kiln. He sad yes, no problem. That gave me the go ahead to do it. I had never seen one as large. I had the advantage of a temperature control engineer, who worked for my dad, and state of the art temperature control apparatus. He advised me that ceramic fiber is always the best choice for kiln insulation. There is no downside that it doesn't retain heat. The controller and elements will supply whatever heat is required. He assured me that brick lined kilns cost more to heat up and take longer to cool down, both to your disadvantage. He also taught me to divide my kiln in to 3 zones, each with their own element, thermocouple, relay, and controller. I used one programming controller and 2 simple setpoint controllers set up as slaves. The theory is simple. if you draw a rectangle, and divide it in thirds, you can see that the outer thirds have 3 outside walls and one interior side. The center zone has 2 outside walls and 2 interior sides. The center zone always wants to be hotter than the outer zones, in a kiln with one zone control. WIth 3, I get even heat at the 3 thermocouple tips.

I have fired sheets of glass that fill my kiln leaving 4" all around, and then cut with a glass cutter, drilled, sawn, and tempered. Any one of these processes would reveal annealing problems with an immediate crack. I can say without a doubt that my kiln anneals really well, firing on the floor. Annealing float and fusing glasses take place at different temperature zones, the float requiring about 80ºF hotter than BE or 96. At a temperature like 1400ºF, the float is stiffer.

[twin vision glass]

This info is so important to the equation. 3 zones makes all the sense in the world and 3 thermocouples controlling everything. Excellent info Bert. I really do appreciate the input for everyone. You come to the table with so much technical information. Leslie

[Bert Weiss]

This info is so important to the equation. 3 zones makes all the sense in the world and 3 thermocouples controlling everything. Excellent info Bert. I really do appreciate the input for everyone. You come to the table with so much technical information. Leslie

I've seen many quite large kilns controlled with a single thermocouple and relay.