View Full Version : 40 lbs Freestanding Gas Furnace

Kazuki Takizawa
03-27-2012, 03:31 PM
I just bought a 40 lbs round bottom High Temp crucible few minutes ago from Pete, and am starting to build my very first furnace. Freestanding, natural gas fired furnace and I am also going to use it as a gloryhole. I have worked at different shops and have some experience doing repairs on their equipment but never built anything from scratch. Is there anyone out there who has experience owning, running, building a small pot furnace like that? I would love to hear any kind of pointers, thoughts or stories regarding design, burner system, insulation, etc. Do you think this size furnace will need 2.5" castable/7" fiber, or can I get away with like 2" & 5"? I am also wondering how much it costs to run something like this. I probably will charge 2 - 3 times a week. I am not sure if I can afford a recuperator at this point. What type or size burner do you recommend.

Thanks so much for your help.


Pete VanderLaan
03-27-2012, 04:22 PM
While awaiting the comments sur to come, I would check out both Dudley Gibberson's book and Glassnotes by Henry Halem. Both address some furnace issues. The main thing is don't let the flame impinge on the crucible, do have a flue. It could be expensive to run such a small furnace as a gloryhole as well. It will be interesting to see what advice you receive.

Hugh Jenkins
03-27-2012, 04:27 PM
A 40# furnace will not use proportionally less fuel than an 80# furnace, maybe 30%, but will be smaller and less costly to build. The internal temperature is what determines the proper insulation profile, so wall thickness should be the same regardless of the furnace capacity.

There has been a lot of discussion lately about optimal insulation profile for cost and efficiency and it can be better than all frax.

You might talk to Lee Miltier about his fuel consumption before and after installing recuperation. That was a very tough retrofit, but still has worked out well for him. Installing when building is the best plan and saves you from throwing away one combustion system when you replace it.

All the current wisdom that I am aware of suggests that firing horizontally above the crucible is the best way to go.

Pete VanderLaan
03-27-2012, 04:29 PM
All the current wisdom that I am aware of suggests that firing horizontally above the crucible is the best way to go.
Totally agree. My concern is over using fiber as a hotface and how that would go at the same time trying to use it for reheats.

Hugh Jenkins
03-27-2012, 04:41 PM
He did not suggest having a fiber hot face. I do think that for a smaller furnace that 2" castable walls are sufficient. The weight load will be way less than for larger units.

There are a lot of shops that use the furnace for a glory hole again. We all used to do that in the old days. Having a changeable door with a heating hole or a set in place tile works way better than a wide open door. The size of the work that can be done well this way is limited, but when it works for what you are doing, it is way less expense than firing up a 14" glory hole to blow cups or goblets or perfume bottles for example. Larger assemblies or incalmo would necessitate a glory hole with such a small furnace.

Pete VanderLaan
03-27-2012, 05:12 PM
I'm aware he didn't propose fiber. I am just wondering about how responsive a small furnace will be and I think you cite the problems well. The problem I always felt when using the furnace as gloryhole was spot heating. In that capacity I am wondering aloud how to get some reflecting heat in the doorway. Fiber came to mind in bringing the furnace up really fast. I think it creates real problems from a durability standpoint.

Maybe just a seriously oversize burner.

Kazuki Takizawa
03-27-2012, 05:22 PM
Hi Pete and Hugh,

Thanks and Mahalo for your input. Those are all great pointers. I didn't know about Dudley Gibberson's book so I think that will be my next step. An 80 pounder will be really nice, but as a starter, I plan to just go really small and make like a goblet and cup shop. 80 pounder will be too much expense for me to even think about building, and even if it is small, I just want to start melting glass on my own. I am renting a studio space in an industrial warehouse so I want to build it fast as well. In the future, I think it can be a color pot.

Im sorry but I think I'm not following you guys with the fiber hotface discussion.. What do you mean by that?

Pete, like you said, I think spot heating is going to be tough too.. I was thinking of making a switchable door with opening smaller than furnace opening so that it can act as a retaining ring.. I am only going to be making cups out of this so the reheating hole does not have to be big.

Tom Clifton
03-27-2012, 05:38 PM
Some specific reason for gas instead of an electric 40# such as the Mark Lauchner design?

Pete VanderLaan
03-27-2012, 05:51 PM
Keep in mind two things: First you will go through 40 lbs of glass in a cup and goblet shop pretty fast. While the furnace would cost more to build, Hugh is right saying it won't cost more to operate. You will get killed on how often you fill the furnace. That's where the operating costs are.

Electric will be a bitch of a cost if you reheat in it and would really suck while trying to do it.

Hugh Jenkins
03-27-2012, 06:17 PM
Kazuki, you will not want a fiber hot face so ignore those comments.

Bob Stephan has run his electric furnace and used it for reheat and claimed it was way less expensive than operating a glory hole. There may be others as well.

My main problem for all except smaller work would be that I want to gather at 2060 and run my glory hole in the 2200-2240 range. I have moved away from spot heating to a much more general heat distribution in the GH and in that way furnace heating could be great.

I have built furnaces with the burner to the back and to the front. I don't think the glass cares, but with reheating as a function, I would move the burner up to the front quadrant.

Cecil McKenzie
03-27-2012, 06:55 PM
Maybe you could put the gloryhole in the flue of the furnace. The gloryhole would then always be a hot to a degree and you could have a small burner to bring it to the temp you want. Use the waste heat to partially heat your small gloryhole.

What are you using for safety controls?

Travis Frink
03-27-2012, 08:39 PM
I regularly work out of a ~100lb furnace like you are considering. Some things I would be considering if I was to build my own are:

Can you make recuperation work or at least get a burner and flue design that will make conversion easier in the future? There are some good designs in Glass Notes 4th edition

I know you are building for a 40lb pot but can you make the furnace interior big enough to accommodate the next bigger size. Charging seems to be one of the biggest concerns with the furnace I use. Charging less often may change your gas bill significantly.

Are you going to be working alone or with another person? Your door design may benefit from some serious design consideration. We use cast rings/discs with different size openings in them but they must be changed by an assistant with a long pair of tongs.

The furnace here uses a series of dampers to direct the exhaust up the flue or through the annealer which also has wire elements for better temp control. The original design had a glory Heated with the exhaust from the flue but we don't use that. I don't know the exact reason for no using the GH in that design but having it preheated with the furnace exhaust would make it much more efficient and faster to heat with a separate burner.

The burner on the furnace here is about 3-4 inches above the lip of the pot near the middle of the furnace slightly towards the back. Spot heating is possible but the punty or pipe gets hot pretty quick. Otherwise there is a nice even heat. If you like spot heating, putting the burner near the front might be a good idea.

The furnace I use has a 10" deep space at the door before you get into the larger space of the furnace (don't know how to describe, but it is like the entryway in a house) that is where most of the heating takes place. It is where frit and powder are melted in so nothing falls in the pot. I would make sure the ledge or sill does NOT extend over the pot so nothing that falls there will drip back in the clear.

It may not be the most hi-tech furnace out there (and could be made a lot more fuel efficient) but a lot of great work is made out of it including lots of cane, murrini, and incalmo in teams of up to 4-5 people.

I can post pictures if you want.

Patrick Casanova
03-28-2012, 10:37 AM
I got 14 years out of my old furnace with the burner coming in below the pot and never had a problem with flame impingement... the bottom of my post is 14 inches off the floor with the center of the burner (Wilton Tip) a little below mid point. (I'm using one of Pete's 225 - 250lb pots and batching SP) Mine is a based off Wes Hunting's design in Henry's first book. The new furnace I built last fall is of the same basic design. From what I have been told over the years the furnaces that had all the trouble with flame impingement were those where the pot sat on the floor and the burner came in to the chamber mid way up the pot wall. My flue is at the same level as the rim of the pot. With my damper I can move the hottest zone from the top to the bottom of the chamber by adjusting the damper, not unlike running a wood stove. Potters will tell you they do the same with their kilns when firing to cone 10 and needing to "balance the temp out in the kiln."

The "Law of unintended consequences" bit me on the design of my new furnace. On my last furnace my flue and burner port were on the left side of the furnace (when facing it), the burner came in tangently and circulated clock wise within the chamber. On my new furnace I moved the burner and flue to the right side (because in my new space I wanted to locate the chimney more in the center of my hood.) This time my burner moves the flow in a counter clockwise direction... doesn't sound like it would make any difference. However it made a huge difference in the amount of "sting" or exhaust that come out of the door when gathering.

The flame coming in under pressure behaves much like water being pumped into the chamber and it wants to find the easiest way to escape. In the old configuration the flame pattern moving clockwise went right past the door when it was open and too the flue. The door opening when gathering was as though it (door opening) was in a back eddy to the flow. The main current to the flow just kept moving as it had been. Now with the burner on the right and moving the flow in a counter clockwise direction when the door is open it gives the flow an easier escape route right out the door because it is within the natural movement of the flow... Not a huge design flaw. I can live with it. It is only a pain when I have to have the door over half way open for a large gather or when charging. But it is definitely a condition to be dealt with that requires a mitt or glove for your front hand at those times. Whereas in the original design there it wasn't an issue at all.

Having now melted just over a ton of glass in it since November the only change I would make would be to the direction of the flow. I would believe the same conditions exist in the chamber if one is firing with the burner at or above the lip of the pot... have your air flow go clockwise! You'll be happy every time you gather.

Pete VanderLaan
03-28-2012, 10:49 AM
I know at this point that Kazuki is upgrading to a 14.5 inch pot.

Tim Elias
03-28-2012, 07:49 PM
i have used my furnace as glory since it was turned on it was designed this way. it also has a ten inch deep throat. this gives a heat line at the door and also at the inside of the throat. we run it at 2150 to 2185 depending on viscosity desired. energy bills and shop temp a lot lower than when we fire up the glory. thousands of goblets have come out of this furnace. very even heat.

Hugh Jenkins
03-28-2012, 10:54 PM
Tim, what do you have to do to compensate for heat loss when heating out of the door? Is there an automatic temp correction or do you turn up a certain amount? What temp do you run at? What is the door opening used?

I actually think more shops could use furnace heating, but they will want some data to work with that helps justify the changes needed.

Having the door open for very frequent gathers requires adjustment with recuperated furnaces because of the heat loss out the door. I would imagine that would be something that would have to be anticipated if there were any constant opening for reheating. This should not be a deal breaker though. Glory holes are big energy wasters, and great room heaters.

Virgil Jones
03-29-2012, 06:04 AM
"Bob Stephan has run his electric furnace and used it for reheat and claimed it was way less expensive than operating a glory hole. There may be others as well."

Bob's furnace is a 350lb corhart tank furnace. I would think the heat mass would have something to do with temp rebound after reheating in it.

Pete VanderLaan
03-29-2012, 06:08 AM
I recall him using much more veiling and solids but no blown ware. Isn't that true?. That would make for a very different reheat experience.

Michael Mccain
03-29-2012, 06:13 AM
I like Ceci'ls notion of capturing the exhaust heat from the flu. That's where you could use a fiber lined cavity to reheat your piece. Remember a good recuperator pays for itself in a year-consider your long term operating costs and weigh them against start up costs.

Thomas Chapman
03-29-2012, 07:44 AM
Bill G. reheats in his furnace, not using a glory hole. He said he likes heating "over glass".

Tim Elias
03-29-2012, 11:57 AM
Hi Hugh
Our furnace is an old moly so the controller trys to keep the heat to set point. when working solo on large pieces when i cannot keep closing door the temp will creep down 20 - 30 degrees. We have a rolling door which will be a third to half open. 9 inch total.My wife chantal runs about 2150 for thicker glass i run 2175. I believe the furnace was designed with extra power to be able to do this.

Virgil Jones
03-29-2012, 12:02 PM
I recall him using much more veiling and solids but no blown ware. Isn't that true?. That would make for a very different reheat experience.

That's right. Less re-heat needs.

Pete VanderLaan
03-29-2012, 12:49 PM
well, if you do reheat in it you have to presume you are eliminating the kill switch function which, while I don't mind doing it for personal use, I would never be willing to do it on a furnace for sale. I find electric reheat to be WAY to subtle ,but gas is OK. I did reheating from the furnace for the first two years I made glass... awful glass I might note. It was an interesting event when a piece came off the punty. Either just really messy or really really messy.

Hugh Jenkins
03-29-2012, 01:19 PM
Oh yeah, losing pieces into the tank was lots of fun. I would like to think I wouldn't do that any more. Teaching situations would make that in inevitability, however.

As with any decision about equipment, this one comes with lots of different opinions and results. We do learn to live with what we have, and we all have reasons not to change what we know until life as we know it is threatened.

Concerning running heat into a glory hole from a furnace; that only saves energy when you are using the glory hole. Recuperation saves energy 24/7 whenever the furnace is on. I now use 1/3 of the fuel I once did and have larger furnace and glory hole. That has allowed for survival when propane is in the $5/gal range again.

There have been some very clever linkings of equipment, crucible in glory hole to garage to annealer. Most work with periodic use not continuous campaigns.

David Patchen
03-29-2012, 01:35 PM
I frequently reheat in our Stadelmelter when I'm making murrine and it seems like a different effect on the glass. I'm not sure how it would work for blowing, but a reheat to a solid piece (murrine) in the electric environment seems like it penetrates a bit deeper without the surface getting crazy hot. This seems like the key difference between radiant heat and a flame. I'll find my murrine can be soft throughout without the surface being super drippy. It's pretty cool but I only use the furnace heat when my assistant is heating things in the glory hole. I don't use it to heat for the pull...mainly because the furnace is far away and involves dealing w/the door.

Josh Bernbaum
03-29-2012, 02:36 PM
a reheat to a solid piece (murrine) in the electric environment seems like it penetrates a bit deeper without the surface getting crazy hot. This seems like the key difference between radiant heat and a flame.

Perhaps a difference between convection/flame and radiant, but perhaps that most of us run our GH's hotter than furnace working temps?

Kazuki Takizawa
03-30-2012, 02:58 AM
Thanks everyone for the great tips. After hearing Pete and Hugh's thoughts, I decided to go with 14.5 in crucible that holds about 80lbs of glass. I have also decided that I need to get a recouperator one way or another. I most likely can not afford to buy one so might have to make one. I only have a MIG welder so making a air tight weld seems a little tricky.

I was surprised to find out that some of you use electric furnace as gloryhole. I thought
Patrick, thanks for your tip on the burner position. I think I will design it so that air flows clockwise too.

Travis, I really like your furnace design of having extra space between the door and the crucible. Yes, I would love to see a photo of it. Is it kind of like a 10inch deep sill?

Thanks, this forum is awesome.

Dave Bross
03-30-2012, 07:32 AM
Years back I think Hugh posted digrams and descriptions, or maybe a link to all that, of how he makes his recuperators. He casts them up out of castable so it's the sort of tech that can be done with more time than money. Quite clever too.

There's another post fairly here recently that goes into gas furnaces and their evolution that has a lot of good tech info in it. Good to know why things ended up the way they did.

Hugh Jenkins
04-01-2012, 04:38 PM
OK. What to say at this point. There are diagrams of the basic operation of recuperators in Glass Notes. All that is written there still applies. I still use compressed air instead of blowers to drive them. I have dealt with glory holes as well as furnaces. Larger glory holes caused some problems with fuel delivery in low pressure systems, but I think that is cured in the newest designs. But, that was the source of the few disappointments.

What has continued to change is the stack design, which acts as the heat exchanger. They started quite simple, got more and more complex, then went to mullite tube, and now have gone much simpler again with stainless tube in tube design.

A few people have gone through the process of building there own from the designs that Charlie Correll and I have published. From what I know many are successful. Most also say that they gave up a lot of time doing it, when it could have been done in a few days and they could be back to work.

I bought Charlies furnace systems when I was teaching, but had my own ideas to try in my own shop. Many years later, I am quite satisfied with the design but the fabrication is still way too hand made.

Charlie could have bought my glory hole system, but he really has a different heat exchanger, and uses blowers and so finally (our inside joke) worked out a design for glory holes as well. Our sharing of information goes both ways and I think we have both progressed from that.

I think one significant difference with what we do is that I really want to work on site, do retrofits, and can really save a lot of time in return for doing the job and adding useable life to existing equipment. Charlie builds complete equipment. He once supplied recuperator kits, but they were not always installed or operated well, and I think he has backed way off from that.

I have to be more and more careful not to feel responsible when someone tries to build on their own and the phone starts ringing. What I have built, I have to support. Otherwise I am just giving time and experience away and I am more and more selective about that. I still want to inspire change and the knowledge that we can all do much better with fuel that we are using. I also know that what I do pays back quickly and in some cases in less time than it would take to go through the DIY thing yourself. I have some very happy references.

So, I come up way short of selling a $29.95 set of plans that will work for everyone, because it just isn't there. It is not a withhold, it is just not a reality.

I recently took a short forging workshop and decided to convert an old glory hole into a forge. It was a fun and successful repurposing of something that was just sitting around. With recuperation it gave great heat, a large capacity and fuel economy.

I am now working on what to do with the 600 to 800 degrees still left in the exhaust after recuperation. There are reasons not to just keep making longer and longer heat exchangers. What is best use here is still wide open and there are lots of good things to try in different places. I am very hopeful about thermal electric generation, but not thinking it is a piece of cake. Hot tubs are a way easier application, but won't come close to using what is available.

Pete VanderLaan
04-01-2012, 05:36 PM
I am totally with Hugh on that. I get requests for plans for building a moly electrical panel and i just can't do it. I build the panel, period. I integrate specific transformers with specific furnace sizes based on the watts that will get used. I work with Charlie regularly. We might as well be called Curmudgeons Incorporated. While I love craftweb it can only go so far down that road. The stuff that has been learned over forty years for Charlie or Hugh or me is all kind of relative stuff when it's applied. The hard and fast stuff is almost never easily explained. I say "Huh," a lot on the phone "Why did you think that was a good idea?" Mary Beth is always amazed at how long I'll talk to people. She hears me say "Huh" a lot. Sometimes Charlie and I say "huh" to each other a lot... I was with Hugh out in Calif at Christmas and when this guy we were talking to about his annealing said something , we both wheeled at the exact same moment and incredulously said "What?"

It's a disease.

Eben Horton
04-01-2012, 07:18 PM
About heating out of your furnace- i do it sometimes, but it will take me 2 to 3 times longer to reheat a piece than in a hot glory hole. Time is money and it makes more sense to make 4 or 5 more pieces a day than it does to save $40 or $50 in gas to run my glory hole. I like to keep my glass cold, so my furnace is usually around 1975-2010

Kazuki Takizawa
06-15-2012, 02:13 PM
Hi Everyone,

Thank you so much for sharing your thoughts and experiences.. Craftweb forum is such a great resource. I got a chance to talk to many (very) experienced glassblowers and shop owners.

My furnace building is progressing very slowly... I am getting ready to cast the walls now and I have another question that I want to ask you!!

Where do you think I should position my flue? and for what reason. My burner is going to be positioned on the right side of the door, slightly above the glass level, and flame going across the surface of the glass.. I have seen flue being position by the back wall of the crown, top of the crown, on sidewall below burner... What do you think is most efficient in your opinion??

Thanks again,

Pete VanderLaan
06-15-2012, 02:34 PM
I would put it in the back wall on the same side as the burner block raised slightly off the floor so a leak won't fill it. Burner block just above glass line is good design.

Little furnaces need as long a path as you can get to allow the fuel to burn completely. When you do flue it, put a decent height stack on it that you can place a brick on top of to control the size of the opening.You could with some effort build a recuperator into that stack. Insulate as well as you can afford to up to eight inches of frax. Anything more than that is diminishing returns.

Rollin Karg
06-15-2012, 04:59 PM
Put the flue next to the burner and slightly below it. That's where you'll want it when you get ready for recuperation.

Pete VanderLaan
06-15-2012, 05:15 PM
Do make sure you give it a big enough chamber for complete combustion. It's not hard to accidentally choke off the burner in a small furnace.

David Palmer
06-15-2012, 07:17 PM
I got here too late,I've been using my setup for 10+ years & it's still going great, so well in fact I'm willing to put my name to it as a product.

I developed a new flue tech' & do a hot air balancing act with simple physics.
Plans available, help turn studio glass into a home hobby.

Kazuki Takizawa
06-15-2012, 11:14 PM
Thanks Pete and Rollin Karg,

Pete, I didn't think about the size of chamber affecting the efficiency of flame. It totally makes sense though. I will have 7in frax and 2.5 in side wall so it will probably be okay?

Pete VanderLaan
06-16-2012, 04:58 AM
That would be fine on the insulation. Years ago I did efficiency testing to look at when you got to the point where adding more frax no longer helped. It was eight inches. Frax cost a lot less then too. If you want frax you can really pack into a space, use the lubed stuff. High Temp in Fenton Missouri sells it but I don't see it regularly stocked anywhere else. Nice Stuff. Ask for Tom, just don't let him start talking about horseshoes.

Rollin Karg
06-16-2012, 07:09 AM
Kazuki are you going to use Natural gas or Propane ? What rate will you paying for your fuel and what rate will you pay for electricity ?

Kazuki Takizawa
06-16-2012, 02:15 PM
Thanks Pete for the frax advise. But what's the difference between lube frax and something like inswool. There is a Harrison walker branch 30 mins from where I live so I was planning on getting that. With frax, should I pack it tight or wrap it loosely to get the most insulation effect?

Rollin, I'm using natural gas. Last time I checked it was like 60 cents a therm. I don't have a big enough propane tank and I want to use the furnace as glory so I decided to go with natural gas.

Pete VanderLaan
06-16-2012, 04:02 PM
Lubed frax is used where you want to pack the material into a closed space to increase the density. Some people wet it and it can be hard to get the moisture out of a serious closed space. Blanket will work out fine. Use 8 lb density, not six which you would regret. Cut it in strips where it won't naturally pack in.

As to our "problem solved " person. Exposed frax is not good stuff and I don't advocate it. While I may have it around my own shop, it's not on my list of things to recommend as a finish product, hobby or otherwise.

Pete VanderLaan
06-16-2012, 04:07 PM
I got here too late,I've been using my setup for 10+ years & it's still going great, so well in fact I'm willing to put my name to it as a product.
I developed a new flue tech' & do a hot air balancing act with simple physics.
Plans available, help turn studio glass into a home hobby.
I've been to the site and I can recognize a hustle when I see one. Here you go folks. For a mere 20K (AUS) you too can be a hero. For those of us well past the 40 year mark in the field, this stuff is an irritant.

Patrick Casanova
06-16-2012, 07:33 PM
Here is my experience with burners on the left vs. on the right, in a furnace with a front door that opens by moving the door to the left. My last furnace had the burner coming in tangently on the left and moving the flow clock wise... think of it like a flow of water. When the door is open for gathering the flow in the chamber goes right past the opening. It is actually difficult for the flow to get out the door opening in that it is at a right angle to the flow. My new furnace I moved the burner to the right side of the furnace, my flue is and was on the same side as the burner. Again I have it going tangently this time counter clock wise. Now when the door is open even just a little it creates a natural escape for the flow to exit. It does so because it is in the natural direction of the flow. Out the door becomes the path of least resistance. I can watch it take place by throwing a small piece of wax in the furnace and watching the swirl of the smoke.

I moved my studio and changed sides when I rebuilt because I wanted to locate the flue more centrally within the space under the hood. I also wanted the extra work space when I put a recuperator on it. If I had it to do over again I would have just put my furnace over to the right and had my Glory Hole on the left.

I'm also a believer in burners below the pot... but only if you leave ample room. I believe you get better combustion with a larger area for your flame to develop. But that's a separate topic.

Pete VanderLaan
06-17-2012, 06:07 AM
Burners below the pot cause hot spotting in the bottom of the furnace. I have measured it again and again and it runs about 400-500F hotter than in the upper chamber. Given that, if you have the upper chamber thermocouple at 2250F, you could expect readings down below to be over 2600F and higher getting to the limits of what gas is going to do without oxygen assist. Alumina crucibles are not capable of sustaining that.Then, if the flame impinges on the pot in any way, you can expect about 30-35 % shorter pot life. I base that on questioning my customers on every crucible sale. Stressed pots interestingly almost always crack on the opposite side from the insult. A classic thermal shock crack is somewhat different and looks like a crows foot ( Industrial slang ) - a horizontal crack with a vertical coming up out of it at a slight angle, usually found at the bottom of the arc in the pot. When Clients do have bottom burners, I advise throwing anything they can find into the flame path to get it off of the pot, which at a certain point is counterproductive if you need a long luminescent flame.

Patrick Casanova
06-17-2012, 08:09 PM
Knock on wood... god I don't know if I'm now asking for it by saying this, but in 15 years and two furnaces I have never cracked a pot. I've had them drill holes. I've had them develop cracks in the lip as the pot got past the point where I should have replaced it but I needed to stay light up. Which was the case with my last pot.... causing my rebuild.

Pet you are absolutely true if you don't know what you are doing that will occur. Especially if one is prone to cranking up the burner to get a quick melt. I've got the capabilities of running two thermocouples on my furnace. Just the act of charging the pot will create a temp differential of 250 degrees between the top thermocouple and the lower one. So for me I don't crank it up full throttle, I leave it and wait for the upper mass to get hot and melt out. By the time I'm done with my melt both are within 10-15 degrees of each other.

For example I just charged yesterday. Approximately 175 lbs of SP in one of your pots. By the time I topped off the pot with the last 20 lbs or so of batch the lower Pyrometer was reading 2300 the upper was at 2285. I threw in the batch and it immediately dropped to the 2050 range on the upper and didn't move down below. If I had cranked my burner up to hit my soak temp of 3225 as quickly as possible, up an additional 20-25%, I could hit it in an hour or less. However the bottom would have gone up into the 2450-2500 range. I'm running a Wilton tip on Natural Gas with maximum combustion air pressure of 12.5 water column inches. For me normal Idling along at 2080 working temp is 2 3/4- 3 ", when charging I'm up to 9.5", to cook I'm at 6-6.5".

It is no different than running a molly furnace or an electric melter. Point is you have to have some understanding of what you are doing and what is taking place in the chamber. Along with a knowledge of what does what. You have to dick with them all. There's no plug and play.

I know a lot of the early "commercially available" furnaces had the pot just sitting a couple bricks off the floor with the burner coming in at about that same height. A lot of early pot furnace's were daytanks with a pot stuck inside. And that was a bad design and I can see where the pot would definitely be impinged upon. The bottom of my pot is 14" off the floor.

He's building a 40 lb furnace... how big is that pot? 40 lbs has to be small. He is going to need a rather large chamber just to get a full burn of the fuel in a space that small. That little pot could sit 2 ft off the floor and it is going to need a good amount of space above the rim as well in order to get enough volume. Too small of a chamber will be more expensive to operate than too large with that small of a pot. To suggest that the burner below the pot is out of the question I don't think is accurate in all situations, especially with such a small pot. Almost all pottery kilns are built with the burners low at the floor... they fire green ware. The have to balance the top and the bottom, you can't have cone 10 on the bottom and cone 12 on the top.

A well thought out and built small gas melter would be the way to for a lot of people in parts of the country where Natural Gas is cheap... Here in the Twin Cities Excell energy is still burning Natural Gas to make their electricity.

Pete VanderLaan
06-17-2012, 08:25 PM
drilling is another variant on the overheated base of a pot.

Franklin Sankar
06-17-2012, 09:21 PM
This is very interesting. I thought the top of the furnace would be hotter than the bottom because the heat raises to the top and acumulates there. Now i know, imagine what happens wen you open the door.

Patrick Casanova
06-17-2012, 09:25 PM
Actually it was half way up the wall right in the area where a typical third charge level has been repeatedly eating away... That area where I top off the pot with out using the bottom 1/3. The Pot wall in general was in good condition on the bottom 1/4 the upper 1/2 looked like cottage cheese.

I showed the pot to Wes when he was here while I was rebuilding. My design started with his second generation. We discussed what our experiences had been, modifications tried and their results, and what I was doing in my rebuild. When the pot was upside down you could not tell where the burner had been. The only way you could tell the front was by the discoloration on the lip from gathering.

Kazuki Takizawa
06-17-2012, 10:32 PM
I agree with Franklin.... I did not know that the bottom of the furnace gets hotter than the top.. and Yes... I can imagine all sorts of things that could happen with furnace door open..

Patrick, I couldn't change the thread topic but I decided to go with 80lbs instead of 40 pot furnace... I bought a 40lbs crucible from Pete and posted this thread on Craftweb... Two hours later I called Pete back and asked to exchange that to a 80lbs pot..and I am sooo happy that I made that move.. Now I am thinking I should've gone bigger, but hey this is my first furnace and I'm working with a budget..

Pete VanderLaan
06-18-2012, 05:38 AM
I think 80 lbs is the tipping point for efficiency on simply running it. The real expense in a glassworks is when you are melting, so melting once a week has always been cost effective in my mind. When you first get started you will build a lot of furnaces as you seek your comfort zone.

Gas furnaces get hotter in the bottom when they're fired in the bottom. What Pat suggests indicates a good bagwall design. The lower chamber gets hotter when the burner is in the lower chamber. "Pinking" of a pot indicates over firing. Allowing the flame to impinge on the pot in any way shortens its life significantly- by 30 percent, sometimes more. I used to run little Charles Taylor and sons pots that lasted about three weeks.

Drilling and voids are different. Pat seems to me to be describing a void and some are worse than others. It's a spot in the casting with an air pocket and it happens and is not completely avoidable. We can't see it going on. When a pot is vibrated, there's about a tablespoon of difference between too wet and too dry a mix of 150 lbs of material. Waiting too long between layers of vibrated material can cause a void as well, but we're talking 30 seconds. Making pots is a major judgment call constantly. Too dry yields voids and too wet yields cracks while being fired.

A drill is a point where everything just got too hot and the viscosity of the contained metal ( silicon, and sodium are metals) just got too low and began to dig in the interstitial spaces in the coarse grained alumina. Silver is a really common culprit doing that, ask Rollin about silver recovery. Lithium above 1/2 percent is a problem in clear glasses. Boron and barium in the same glass is a major problem. You might as well dump HF in there. Drills are usually in the bottom three inches.

There is this school of though that says "melt hotter" Seattle does this a lot. "If you melt at 2400F, I'll melt at 2450F". Those guys hate everybody's pots. We hate talking to them. They make the boat payments though . The 1979 Corning Protocols as we called them said that you should melt as cold as you can get away with. The colder you melt the higher your linear expansion will be. You will periodically see testimonials here from people who throw SP87 into a furnace at 2000F and get good glass. That is really on the low end of cold in my mind and I would only try it in someone else's furnace. I can see it with cullet though. I melt my own batch at 2200F for the most part and go up to 2300F to complete the reaction. I hate the term "cooking it". It's not a lasagna.

Kazuki Takizawa
06-18-2012, 09:40 AM
Now I know what drill means.. Haha, lasagna!

Hugh Jenkins
06-19-2012, 04:05 PM
I have never liked the term "squeeze" relating to glass melting. But, I have accepted that as the common word for the cooling stage after the high temp melt. It isn't a pillow either!

Pete VanderLaan
06-19-2012, 04:38 PM
What's wrong with "My main squeeze"? OK valence switch then....

Allan Gott
06-22-2012, 10:31 PM
Holy smokes.....Mark........did you find that button yet??????

Pete VanderLaan
06-23-2012, 05:39 AM
.............I did.

Rollin Karg
06-23-2012, 03:24 PM
.............I did.
Thanks !!!!!

Pete VanderLaan
06-23-2012, 05:02 PM
To quote Marvin Lipofsky many years ago:

"Like a hooker screaming in a car window at a traffic light."

Steve Stadelman
06-23-2012, 06:15 PM
A turn of phrase that covers so many situations :)

Travis Frink
06-24-2012, 03:52 AM
"Like a hooker screaming in a car window at a traffic light."

Sometimes I'm REALLY glad I live in Japan...!-)

And sometimes I'm not....

Pete VanderLaan
06-24-2012, 05:22 AM
well, this was international in flavor.

Travis Frink
06-24-2012, 08:23 AM
well, this was international in flavor.
I've had "ethnic" flavor but am suspicious about the" international" - especially as it is sometimes used in the same way as "exotic" is sometimes used in the US. I find "authentic" best. Imagine the day when we see signs that say "authentic" dancers... and it turns people on.

To make this post somewhat relative to glassblowing:
Does anybody remember seeing a sign in Seattle on an old style marquee-type sign(maybe on the south side of pioneer square on 1st or 2nd) that said,
"Have an erotic day"

Kazuki Takizawa
06-24-2012, 10:02 AM
"Have an erotic day"

Travis, this is still not related to glassblowing at all!

Pete VanderLaan
06-24-2012, 10:56 AM
They probably meant erratic. Where I lived in New Mexico, Authentic Indians were the norm.

Rosanna Gusler
06-24-2012, 12:34 PM
Travis, this is still not related to glassblowing at all!sure it did. he said seattle. rosanna

Greg Vriethoff
06-24-2012, 12:43 PM
Does anybody remember seeing a sign in Seattle on an old style marquee-type sign(maybe on the south side of pioneer square on 1st or 2nd) that said,
"Have an erotic day"
That would be the (now defunct) Lusty Lady (http://en.wikipedia.org/wiki/Lusty_Lady)

The Seattle Art Museum is directly across the street. When Dale did his first installation there the marquee said "Chihuly Does SAM."

Travis Frink
06-24-2012, 06:12 PM
Rosanna is right. And I will further the idea to include Chihully too;-)

Thanks for putting a name on that memory. Interesting that the SF location is mentioned in a book I pulled off the public library shelf here last week and am reading now- including the details down to the protests for higher wages and even the $27/hr mentioned on Wikipedia.

Were the lusty lady not now defunct, it would no doubt now read:
"Chihully does the space needle"