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  #26  
Old 09-06-2009, 08:42 PM
David Hopman David Hopman is offline
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That's strange, because you'd think that with the ends of the elements sticking out of the back of the furnace, that at some place on them there would be a temp of 1200. I guess the lowered overall temp moves that 1200 point from the solid part to the spiral part and allows a crack. Taking the elements down to room temp doesn't seem to cause problems- 3 of mine have been cycled 5 times already with no issues.

Come to think of it, I used to lower the glory hole to 1590 at night. Stopped doing that because of some issues with the superstructure, but the elements seemed unaffected. Did that for at least a couple of years.

I didn't think that preheating the pellets was much trouble at all. I just filled up a stainless steel tray with 40 pounds of pellets, brought it up in the annealer, then dumped it in. It also reduced the temp loss in the furnace on a charge. I haven't been preheating the batch I'm using now, and have had no problems in 9 months.

Last edited by David Hopman; 09-06-2009 at 08:46 PM.
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  #27  
Old 09-07-2009, 04:26 PM
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Quote:
Originally Posted by David Hopman View Post
That's strange, because you'd think that with the ends of the elements sticking out of the back of the furnace, that at some place on them there would be a temp of 1200. I guess the lowered overall temp moves that 1200 point from the solid part to the spiral part and allows a crack. Taking the elements down to room temp doesn't seem to cause problems- 3 of mine have been cycled 5 times already with no issues.

Come to think of it, I used to lower the glory hole to 1590 at night. Stopped doing that because of some issues with the superstructure, but the elements seemed unaffected. Did that for at least a couple of years.

no spiral, these are solid bar type TW. the word from engineering was that the 1200F exposure for long enough will cuase the breaks. And since these would sit for a week or more at that point they were getting stressed. When running the temps fluctuate and move around, same thing when charging. Keeping the furnace static at 1750F, with this particular furnace causes the elements to develop micro fractures 5" inside the walls of the furnace.
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  #28  
Old 09-09-2009, 04:06 AM
Brent Hickenbotham Brent Hickenbotham is offline
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Quote:
Originally Posted by Lawrence Ruskin View Post
If you don't preheat your cullet that glass will go winging around the inside of your furnace.

I would expect a good solid hit on a moly element would break it, but I really don't want to find out.

Unless someone can come up with another solution, I will preheat until the sad little day I die.
I haven't heard of one person breaking a moly element from cullet or anything else popping on it. If someone has, let me know. Those elements really take a beating in a glassy environment, barring the use of flourines.
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  #29  
Old 09-09-2009, 07:21 AM
Doug Sheridan
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hmm...I did break one by bumping it while charging. By beating you mean, works great as long as one doesn't nudge them? Also, since I'm about to change a crucible, are they more fragile after they've been hot for a year?

Last edited by Doug Sheridan; 09-09-2009 at 07:22 AM. Reason: typpo
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  #30  
Old 09-09-2009, 03:01 PM
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They will change shape, more shaped like a flattened wisk so you really need to be careful pulling the passage brick out of the crown. The brick may well be sort of stuck in the crown as well, mine frequently do. Have a place absolutely clear to accept the element and the brick when you pull it. Make sure you have material there to chock it all up as a support.

The only times I have ever broken an element is when I'm changing a pot. I've done it more than once.
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  #31  
Old 09-09-2009, 07:42 PM
Brent Hickenbotham Brent Hickenbotham is offline
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Doug, I was strictly speaking furnace environment and off gassing. Yes they are still fragile, but handled with care last a long time. Good speaking with you earlier.

Quote:
Originally Posted by Doug Sheridan View Post
hmm...I did break one by bumping it while charging. By beating you mean, works great as long as one doesn't nudge them? Also, since I'm about to change a crucible, are they more fragile after they've been hot for a year?
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  #32  
Old 09-12-2009, 06:15 AM
Rollin Karg Rollin Karg is offline
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Quote:
Originally Posted by Pete VanderLaan View Post
Have a place absolutely clear to accept the element and the brick when you pull it. Make sure you have material there to chock it all up as a support.

The only times I have ever broken an element is when I'm changing a pot. I've done it more than once.
When we finished our first Moly I had a little shot of energy and built a small wood cart on wheels. It's positioned next to the furnace at pot change and stores the elements vertically just like they are in the furnace and in the same sequence. It's been pretty handy.
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  #33  
Old 09-12-2009, 06:41 AM
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Quote:
Originally Posted by Doug Harroun View Post
At first I considered electric elements. Being electrically inclined the wiring and power is a non-issue for me. SiC was out of the question. I know a few people using them and the replacement cost of shot elements would run higher than the cost of gas. Moly elements look better, but the _VERY_ high initial investment was the factor that pushed me back towards gas.

Also, unless you live near a wind farm or hydro plant you power is most likely coal/gas/oil based anyways, why deal with a loss in conversion?

Right now I pay about $0.015-$0.02 per kWh for gas. For electricity it's $0.1085 per kWh.

So I'm still pretty sure I'm building a gas furnace.

...unless someone knows a source for moly elements for less than $100 each.
It's a real good thing to survey your environment when considering whether to go electric or gas. Especially paying attention to how the electricity is billed. Can you say Max Demand and Power Factor ? Natural gas prices at the wholesale level are way low right now. Will they say there ? Is your gas supplier passing on the reductions to you ?
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  #34  
Old 09-12-2009, 12:52 PM
Brent Hickenbotham Brent Hickenbotham is offline
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Quote:
Originally Posted by Doug Harroun View Post
At first I considered electric elements. Being electrically inclined the wiring and power is a non-issue for me. SiC was out of the question. I know a few people using them and the replacement cost of shot elements would run higher than the cost of gas. Moly elements look better, but the _VERY_ high initial investment was the factor that pushed me back towards gas.

Also, unless you live near a wind farm or hydro plant you power is most likely coal/gas/oil based anyways, why deal with a loss in conversion?

Right now I pay about $0.015-$0.02 per kWh for gas. For electricity it's $0.1085 per kWh.

So I'm still pretty sure I'm building a gas furnace.

...unless someone knows a source for moly elements for less than $100 each.


You can Idle an electric furnace lower than a gas furnace, speaking in consumption terms.

How did you come up with the conversion of KWh for gas costs?

After shipping and customs clearance, even buying in large bulk from China, molys are still more than $100. But they last a long time if taken care of.
SiC can last a good time as well in the right settings.
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  #35  
Old 09-14-2009, 03:18 PM
Charles Friedman Charles Friedman is offline
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[quote=Brian Gingras;82323]no spiral, these are solid bar type TW.


Brian, Not to split hairs, but I think the TW in your bar type, means Thin Walled. Mine are TW as well and I can see all the way thru, to the other side of the furnace.
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  #36  
Old 09-15-2009, 10:01 PM
Jim Engebretson Jim Engebretson is offline
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Brent,
I'm pretty sure we broke two elements by charging recycled cullet on top of batch. Both broken elements showed up the day after charging.This is in a 600lb. stadelmelter. We've halted recycling until we can process the cullet (quench) to a smaller, less explosive size.
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  #37  
Old 09-15-2009, 11:31 PM
Ben Solwitz Ben Solwitz is offline
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You could also preheat the cullet in an annealer.
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  #38  
Old 09-16-2009, 02:13 AM
David Hopman David Hopman is offline
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I preheated all the pellet/cullet charges for 8 years- it's not a big deal to do it. Stainless restaurant steam table pans make nice containers to heat in, and are about $25, so reasonable. One 4" deep pan holds about 40 pounds of pellets, a 6" deep one will hold a lot more.
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  #39  
Old 09-16-2009, 07:03 AM
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Glenn Randle Glenn Randle is offline
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Seems you could sort out the largest chunks of cullet to preheat, saving you the bother & time of preheating most of cullet. The smaller pieces don't explode like the baseball+ sized chunks do.
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  #40  
Old 09-22-2009, 02:07 PM
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[quote=Charles Friedman;82430]
Quote:
Originally Posted by Brian Gingras View Post
no spiral, these are solid bar type TW.


Brian, Not to split hairs, but I think the TW in your bar type, means Thin Walled. Mine are TW as well and I can see all the way thru, to the other side of the furnace.
yes thin wall, high density, solid as in not spiral type, but yes they are a tube.
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  #41  
Old 10-06-2009, 01:58 PM
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Worst furnace build thread ever! The only thing I learned is that Doug has the cheapest gas on the planet. Maybe I'm just jealous.
So since this hasn't gone anywhere is it time for it to start it's slow decent off the front page?
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  #42  
Old 10-06-2009, 03:25 PM
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It's the type of thread fraught with peril. The promised documentation failed to appear. Leave it alone. If it gets any quality junk in it, I'll edit out the superfluous stuff.
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  #43  
Old 10-08-2009, 11:42 AM
Lawrence Ruskin Lawrence Ruskin is offline
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It should be rolled over into a thread that reads:

How to build a low cost, low overhead studio.

So that these ideas will be easy to pick out of the archives
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  #44  
Old 10-08-2009, 12:04 PM
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curiously, I can't edit a thread's name at all. I can move stuff from one board to another board and I can cherry pick posts but it is time consuming and spare time is not something I really have. It's easier to delete superfluous stuff. ( which is usually my contribution)
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  #45  
Old 01-05-2010, 04:40 PM
Jim Bowman Jim Bowman is offline
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Basic Question...

I am building a gas fired, freestanding 300 lb crucible furnace. My question is about the platform / base.

I am starting with a 3" x 3" x 1/4" angle iron frame mounted to 4" steel casters. 3/16" plate steel rests in this frame, and the bricks are are layed on the pale steel. I was thinking of doing a one inch layer of ceramic fibre insullation board on the bottom, then a layer of 2300 soft brick, then a layer of hard brick. (Clippers)

Henry's book shows a layer of hard brick, then alayer of 2300 soft brick,then another layer of hard brick. Is the bottom layer of hard brick necessary? It seems the insullation board on the bottom would conserve more heats.I want to keep the gathering port as low as I can, and that would shave an inch and a half off the hight.That would add up to37" as the hight for the sill of the gathering port. On my old furnace, the sill was at 33", and I was comfortable with that hight.

Any comments or suggestions on the make-up of the base would be appreciated. Also is there any standard for the hight of the gathering port?
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  #46  
Old 01-05-2010, 04:55 PM
Slate Grove Slate Grove is offline
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Jim:

It sunds like your plate of 3/16' steel is one solid piece of plate...is this correct? Have you used this design before, and if so how did the heat effect this plate with regards to heat expansion? I would think that with a big plate like that, the heat would cause some weird warpages, but I may be way off base (pun intended),

Here's a good link to check out a steel base (futon) that works really well and is public domain on the spiarl arts website...http://www.spiralarts.com/tech/docum...n_drawings.pdf

It would seem more logical to me to have the steel base, then soft brick, then 2 layers of hardbrick, instead of softbrick sandwhiched between two layers of hardbrick.

There's actually really good documentation of the 1000lb. tank furnace rebuild at Corning on the spiral arts website in the tech section. I know you're building a freestanding pot furnace, but there may be some stuff documented there that will help you out.
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  #47  
Old 01-06-2010, 10:53 AM
Jim Bowman Jim Bowman is offline
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I have used that base design before, and it worked OK. Maybe for a furnace three times bigger that what I'm doing, a sturdier base would be in order. Thanks for the link, though, There is some really good information there.
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  #48  
Old 01-06-2010, 09:48 PM
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Jim.... I was curious if you were replacing your moly or just adding a new one to your capacity?

My schedule for insulation starting at the bottom was 3 inches Insblock, 2 layers of 2300 F soft brick, one layer of hardbrick, 3 inches of cast mizzou. I think this was similar to a noted furnace builders schedule circa late 80s. I think his schedule used 1800 F brick for the lower level of soft brick which would make sense because they are better insulators I believe. I used the 2300 F softbrick because I had salvaged them.

Where my crucible sets I believe I made solid all the way down on the recommendation of a friend whose pot tipped over when the softbricks dissolved after a leak.
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  #49  
Old 01-07-2010, 07:13 AM
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I have a real problem with putting material under something that weighs 2000lbs that is soft like Insblock or fiber. I think it is just asking for a shift off of level. I can see building it with an outer foundation load bearing wall and filling in the central area with better insulating materials, but not ever using them in a load bearing part..

Floors of furnaces are the most neglected part of them. It is a huge face for heat loss and it is never in our consciousness since we can't see it or feel it. It's there. It needs at least eight inches of insulation.
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  #50  
Old 01-19-2010, 11:16 AM
Kyle Gribskov
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I'm sold on moly's at this point. One thing I've turned to in building a furnace is to give up on the clean out port. Sounds daft I suppose; but I get so much time out of my free standing pot in a moly furnace that I prefer to chip out any leakage when pot change occurs rather than take the chance that unnoticed spillage or leakage can invade the joints of a furnace built of various parts either dry assembled or cemented. I think those folks who are sticklers for cleanliness and frequently check their clean outs for accumulation of glass can get along fine with any set up. Those who don't check frequently might want to consider a big cup to contain the crucible and avoid a big rebuild. I've gotten 3 years as a rule on my EC cruces and usually get to them before they go away; but when I haven't, the monolithic casting I use in the bottom seems fairly bullet proof. I've had the same one in the furnace for 12 years with no leakage.
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