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Don, questions about firing Differences between Hoaf and Electric Kilns
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 Posted: Thu Jan 29th, 2015 09:50 pm
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Cameron G
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Don,

I was examining the firing photos from your web site. I noticed there is a significant difference between the Hoaf firing and the electric kiln firing. I'm referring to the "maturity difference" you highlighted.
 I'm considering buying a Hoaf, but didn't realize the results would be so distinctly different. It looks to me like I'll still need to run pieces through my electric kiln to get a good finish (and perhaps a better anneal).
Can you tell me how the Hoaf fits into you process? What it's good at and what are its limitations?
I'd like to know a bit more about the utility of the kiln, with the $7,000 to price tag.
Any insight will be much appreciated.

Cameron



 Posted: Fri Jan 30th, 2015 07:53 am
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Don Burt
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Cameron,
I have some experience and have some hypotheses about these topics, but I don't have proof or expert opinion to back it up. I can tell you what I suspect though:

The differences between the results of paint fired in a Hoaf as compared to an electric kiln are the result of the amount of 'heat work' the glass undergoes, rather than any big difference in the atmosphere or evenness of the radiation. One thing that is a factor here is that my primary electric kiln is a top-element Skutt clamshell, which fires very evenly straight down upon the kiln shelf. Side element kilns are not as even in firing flat glass, so they potentially can not be set to ramp-up the temperature as fast as a top element kiln. I believe I could fire my top element skutt at full blast and not break glass. I never do it though. The time savings of the Hoaf is not only in it's ability to reach process temperature in about 10 minutes, but also in that it cools so much quicker than a brick electric kiln. You usually can retrieve your work with in hour.

If I fire a piece of 3mm thick painted work in my hoaf, I usually take it to about 1275F and then shut it off. If I fire in my electric, I ramp it up via its control at say, 600°F per hour to 1275, and I may add a few minutes hold time at that top process temp. In the electric, then, the paint spends a lot more time at temperatures above the glass strain point, than it does in the hoaf. The hoaf has very little insulation and has vent holes around its entire perimeter, so it cools much faster than an unopened electric kiln made of typical firebrick...so the glass ultimately incurs more heat work in the electric kiln.

Now I can manually accomplish the exact same heat work in my Hoaf, but I'd have to stand there with a lever in my hand turning the gas on and off while watching the pyrometer. As I mentioned above, I can fire faster in the electric but I don't....I think it can actually get to 1300F in about 20 minutes. I can do things with propping the kiln lid open on the electric, and make it cool faster, but I distrust the risk of the drafts causing strain to the glass and the bricks and elements....besides, it's troublesome and boring to do either of the manual variations.

In my enamel samples, I think that the variation in color on the reds and purples are simply due to lower heat work in the Hoaf. If I would have held them at top temperature longer, they would have been more like the electric results. I have had some problems with bubbling of the enamels in the Hoaf, but I've come lately to the conclusion, after having done some experimentation, that those bubbles were caused by the bed of whiting that I fire on. Whiting has caused problems with enamel in all of my kilns. Even so, I just don't understand the transparent enamel behavior in the Hoaf well enough to trust it. I trust it in the electric, so that's what I use.

For opaque paint...i.e regular Reusche glass paint and silver stain, I think the Hoaf does fine, if you take into consideration the amount of heat work and compensate accordingly so that you get the gloss you want. The pyrometer indicator on the Hoaf is only useful for noting where you got the results you want. Just because Reusche says their color matures at 1250°F, doesn't tell you how long it has to stay at or above that temperature to get a desired finish. Start with 1280-1300F° and adjust from there on the Hoaf.

So "Why would I want to get a Hoaf"? Well, so you can fire and remove your work in one hour, for a cost of about 75 cents per firing.

"What if I buy a Top-element electric, and fire at top speed to process temperature, and then prop the kiln open so that it cools in say, two hours?" Well, that might work, but its easier to accomplish in the Hoaf, and the Hoaf with the lid down cools more evenly than a propped lid electric, I'll wager.

"Does eveness in cooling, and does annealing really matter on 3mm glass"? - This is a tough question. All glass manufacturers anneal their 3mm glass. But do they anneal it more thoroughly than you would accomplish in a lid-shut Hoaf? I don't know. I have never heard of anyone complaining about stress in their stained glass from firing it in a Hoaf. Graham Stone in his 'Firing Schedules For Glass' states that 3mm glass only requires about 5 minutes of annealing. I've never timed the cooling of my Hoaf in the annealing range of 1000-800, but it seems to me that it might go slow enough to accomplish that manufacturer annealing specification. But my guessing doesn't conclusively answer the question as to whether the Hoaf will introduce strain. And there isn't a perfect answer as to how much strain is too much, and what effect it has on the durability and usability of the glass. Maybe a little stress could actually toughen the glass from certain types of compression or tension...hell I don't know.

I really think there is some potential for improving electric kiln design so that it can cool more quickly: add more vent holes that can be evenly opened, or devise a lid that can be propped open a bit evenly all around. And by reducing the thermal insulation, using fiber products rather than kiln brick.

Let me try to answer your question from a more practical side: If I was a professional glass painter that had to meet deadlines and fired daily, I would want a Hoaf, even at $7000. And I'd try not to worry about the annealing, because ignorance is bliss. For transparent enamels, I'd still use my electric. If I wasn't time constrained, and I painted all day and threw everything into the kiln at the end of the day for retrieval in the morning, there would be no additional value in having a hoaf. I'd prefer the electric because you set it and forget it. Two electric kilns with controllers would cost about $5000. You could fire one while the other is cooling, with a lower investment than the Hoaf. But you'd always be aggravated waiting for that first firing to cool.



 Posted: Wed Feb 4th, 2015 01:02 pm
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gil
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Cameron and Don,

My first experience with a glass kiln was in 1974 with a gas "flash kiln" at Appleton-Rubec Studio. There is an illustration of one in Albinas Elskus book "The Art of Painting on glass. The single shelf, a sheet of pure asbestos was about 24" X 36" but I'm just guessing. Rambusch still has one of these, but I don't believe they use it anymore. Durhan Studio still uses one to this day. It's the one that Albinas used when he was an owner at Durhan in NYC. I learned to fire on that kiln. It was stirictly based on appearance of the paint surface, how the flames reflected showing gloss or lack of it. It took about 15 minutes to reach firing temperature which I believe was around 1250F. That's also a guess since there was no pyrometer. The kiln had approximately 12 jets on each side which were pointed at each other just above the glass. There were 4 vents or stacks on the top, and annealing consisted of shutting off the flame and covering the stacks with small asbestos rectangles. The kiln was actually quite drafty, and in the case of Durhan's Studio on 18th Street in NYC, it was outside. In the winter, the "annealing cycle" was well under an hour though it was usually longer in mild weather.
Appleton-Rubec took a more production oriented approach to annealing. We had a good number of shelves, 10 or so, and we would load them all with glass ready to be fired. Once the kiln was lit they would go in one after anther with no preheating, each firing taking 15 to 20 minutes. A hot shelf would come out and placed on a metal tray and covered with a light weight sheet metal hood slightly larger than the tray and around 6 inches high. Each tray was fired in turn and stacked on top off the previous tray till all were fired. The kiln fired continuously through this process. When the glass was cool enough to handle it was considered annealed. Peter Appleton, one of the owners handled this amusing process. I had no part.
In my studio now I have 2 Denver Glass Machinery kilns, a KL-27 with inside measurements of 21" X 21", which I've had since 1989, and a KL-60, 30" X 60" inside which is 5 years old. Both are top element and fire very evenly and quickly. 1250F is achieved in around 25 minutes in either kiln. That is the way I have always fired them, at least for painted glass. They will take around 2 to 3 hours to reach 100F if they are not opened and that is what I generally do, although I do not hesitate to vent them in order to be able to fire every 2 hours. I've never had any problems (breakage) with them. Using a propane fired kiln in a commercial space in NYC would probably not be permitted. Also, the KL-60 costs around $5,000 and both come with avery reliable computer control. Personally I'd rather save the $2,000 and wait and extra hour with a kiln that has more than twice the capacity (KL-60) of a Hoaf, but as they say, to each his own.

Tom



 Posted: Wed Feb 4th, 2015 08:01 pm
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Don Burt
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Having both a 21x21" kiln and a 30x60" kiln is a nice setup. One could still wish for more. That T-shirt about guitars I saw recently comes to mind:

"I have too many kilns" - Said no one ever.



 Posted: Wed Feb 4th, 2015 08:30 pm
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gil
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You're right, I'd like to have a deeper one for casting...................



 Posted: Sun Feb 8th, 2015 04:03 pm
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Hallie Monroe
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When i chose my Hoaf Speed burn it was primarily for painting, I was doing stenciled glass for a restoration. I needed to fire a lot of square feet asap! Also electricity is very expensive at the end of Long Island. I have it vented in my private studio. I run it from a standing propane tank that I can take and refill.  About 7 cents a fire. I love it. The turn around time on the loads is about 1 hour if I crash it at 1250f* down to 900f*. But I play it safe, usually let it take it's time, about 2 hours to opening.  The only glass that has breakage due to thermal shock is etched green flash glass. 1/4" slab too can be ticky.
I also ran a big Denver top fire clam while painting at Willet Hausers these past 4 years. It must have been an electric pig because it was so big( 6'x 36"). I always tried to run with a full batch of cookies. I know the speed burns leave more stress in the glass so sometimes I would run the Hoaf for all the tack fires and multi paint layers, then run the last  paint fire through the electric to anneal the glass.
Also there in a big difference between firing enamels in the Oxygen rich hoaf. The air sucked into the side ports. Compared to the Oxygen starved Electric with longer ramp time.
I keep a index of paint samples run in both electric and gas. The result for enamels is usually better in the electric. I have not seen a difference in Silver stains.



 Posted: Sun Feb 8th, 2015 05:13 pm
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Don Burt
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I'm skeptical about the oxygen starved electric kiln thing. But I don't have the means to measure the differences.

I just now looked up something: 'the normal atmosphere in electric kilns is neutral or slightly oxidizing, and it is better to fully exploit this situation than to attempt the more contrived business of setting up a reducing atmosphere'. ( Robert Fournier from Electric Kiln Construction for Potters.)

Also in a book by the late Gene Patterson called A Book For the Curious Painter he states: "Reduction kilns are usually gas-fired kilns and the reduction of the oxygen inside the chamber is produced by either increasing the amount of unburned gas, or by adding some substance such as moth-balls (Paradichlorobenzene) into the chamber to bind all the available oxygen.
Electric kilns are the most common type of kiln for pocelain painters and they should not be used for full-chamber reduction firing, because electric elements (heating wires) of these kilns can be damaged by heavy reduction firing. However, your electric kiln will not be damaged by firing prepared overglaze lusters becuase the mild reducing agent acts only within the thin layer of luster and cannot damage the kiln's electric elements."

All of which proves nothing other than that I seem to have time to look up things like this and retype them. But it makes me question whether we know what the atmosphere differences are between the Hoaf and an electric, and what its effect on enamels is.

I think we all can agree that we shouldn't fire mothballs in either kiln.



 Posted: Sat Feb 14th, 2015 06:51 pm
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Cameron G
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Hallie. It looks like your experience with the Hoaf and enamels is similar to Don's--which prompted my question. Thank you for sharing your experience on processing your glass with a Hoaf kiln in the mix.

Cameron



 Posted: Sat Feb 14th, 2015 07:01 pm
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Cameron G
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Gil,
I hadn't considered the Denver Glass kilns. I didn't realize they could cycle that fast.
You certainly do get a lot more glass firing space for the money with the Denver Glass kilns, as compared to the Hoaf.

It's 4 degrees Fahrenheit out today, with a wind chill of -21. My Evenheat GTS-2541 kiln is demonstrating one of it's finest features-it's helping heat my studio!

Thanks for the insight.
Cameron



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