looming deadline: how fast can I fire

lohman · Post by **lohman** » Wed Nov 19, 2003 3:59 am

I'm firing a ceramic lustre on double strength float. Pieces range from 14" x 39" to 13" x 28".
I typically fire at 10 degrees F per minute up to 1100 F and then 20 degrees per minute to my target which of course varies.
How fast can I go to 1270 degrees F and do I have to follow my typical anealng and cooling schedule: Cool as fast as possible (natural cool) down to 960 degree aneal, hold one hour and continue natural cool.
Today's lustre firing took a full 12 hours but I need to kick it if I'm going to make my deadlines.

Thanks in advance.

rosanna gusler · Post by **rosanna gusler** » Wed Nov 19, 2003 6:58 am

i think you should anneal around 1030 f for float. rosanna

lohman · Post by **lohman** » Wed Nov 19, 2003 9:21 am

Thank you Rosana. My anealing schedule has worked well at 960, I will try your suggestion. But my question is how fast can I take one layer of double strength to 1270. I can still anneal at your suggested temp.

Tom White · Post by **Tom White** » Wed Nov 19, 2003 10:17 am

In general the limiting factor is how fast you can heat the glass without thermal shocking it. 10 degrees per minute = 600 degrees per hour (my controller works in degrees per hour not minutes). IMHO this is a fairly fast rate of heating and I think that the glass on the shelf is lagging slightly behind the temp indicated on your controller. Once the glass is up to 750 - 800 F it is no longer possible to thermal shock it. As soon as you think the glass is above the strain point temp (perhaps 900 degrees indicated on the controler) you should be able to heat it as fast as your kiln can go to your desired temp. Remember that the faster you increase the temp the more your glass on the shelf is likely to lag behind the indicated temp so you might need a slight soak at your final temp to make sure the glass has reached the desired temp. I have not fired items as large as you mention so please consider this when you evaluate changing your firing schedule. As I understand anealing any time you heat glass above the anealing point (1000 to 1030 F for float glass) it is necessary to aneal it to relieve the stresses in the glass. The only other way I can see to shorten your firing cycle would be to flash vent your kiln to around 1000 F after reaching your process temp. There are as many opinions about flash venting as there are fusers out there. One last thought. could you consider venting (propping the lid or door of your kiln slightly) once it cools to around 500 F? At this temp the glass is once again subject to thermal shock so the venting would need to provide uniform cooling without cooling one edge of the glass faster than another side.

Best wishes,
Tom in Texas

charlie · Post by **charlie** » Wed Nov 19, 2003 10:25 am

lohman wrote:Thank you Rosana. My anealing schedule has worked well at 960, I will try your suggestion. But my question is how fast can I take one layer of double strength to 1270. I can still anneal at your suggested temp.

when they temper 1 layer of double strength, they go from room temp to 1200 in about 10 minutes. of course, if there's any bubbles in it, you risk exploding the sheet.

lohman · Post by **lohman** » Wed Nov 19, 2003 10:26 am

Thanks Tom. I do flash vent down to near anealing temp.
I think I will do a test based on my intuition and experience.

Tony Serviente · Post by **Tony Serviente** » Wed Nov 19, 2003 10:49 am

The factors to consider are how evenly are you heating the glass. There is no limit to how fast you can heat a sheet of any given size if you heat it uniformly. The surface area is one thing to think about, and if you have a top fired and the glass is away from the corners, or any other cool spots that is advantageous for rapid ramp up. The other thing is thickness, and shelf material. You can uniformly heat the top, but the bottom layer may be lagging because the shelf is cooler, and is being shielded from the heat by the glass, or the glass is thick. Since you're dealing with single thickness, that's good too. I don't know how far you can push it, but I've gotten away with heating square foot, single thickness pieces of art glass at 100 deg/minute. After they cool to 900, I vent them and pull out when they're 300-400. Lots of details to this I'm leaving out, but just trying to give you a sense of what I've done in terms of fast in, fast out. If I were you I'd get some test sheets to sacrifice before trying real thing. Wear good dry cotton gloves with no holes! Good luck.

Bert Weiss · Post by **Bert Weiss** » Wed Nov 19, 2003 11:25 am

Lohman

You don't say what style of kiln you are using or how the glass is situated, on a shelf or on a bowl mold. If it has only top elements and the glass is on a shelf, I would take 45 minutes from 100 to 1000. If it is on a not flat mold, I might double that time. For me these are conservative times. If you have side elements, I don't kjnow the answer, but it is slower.

You are likely totally wasting your time soaking at 960. I soak at 1000 which is closer to the lower end of the range. For double strength float glass you don't need to soak at all. Just let the kiln cool at it's natural rate. Crack the kiln between 450 and 300 and after a few minutes, you can crack it some more.

When tempering 3/8" float they heat up to 1100ÂºF in 5 minutes. The huge difference is that they are heating from both sides, evenly. We are not.

lohman · Post by **lohman** » Wed Nov 19, 2003 3:45 pm

Thanks everyone for the feedback.
I'll test-fire a sacrificial sheet. I have a kiln with elements in the lid and can ramp up very quickly. Probably straight up to 1270 then OFF, no hold or soak. I'll flash-vent to 1000 F and then let the kiln cool naturally. I'll crack the lid a bit at 350.

I'll be bedding the glass on kiln shelves butted together. On top of the shelves I will spread a layer of crushed prefired plaster and screed it to level out uneveness. Then a layer of 1/4 inch fiber blanket dusted well with diatamaceous earth/dry plaster mix. then the glass. The elements are about 8 inches above the glass.

I'll report my progress.

Thanks again.

Barbara Cashman · Post by **Barbara Cashman** » Wed Nov 19, 2003 8:30 pm

You should be able to get to 1270 within the hour (in a fiber kiln, especially). I fire 2 layers of Spectrum to 1400 in 45 minutes. Of course, I'm an old, maverick fuser, so you may not even want to listen to me. Fire fast as you want for testing. Every kiln will be quite different. Also depends on your shelf. Fiber shelves will take a temp difference MUCH better than ceramic. - Barbara