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Sizing a Kiln

I am looking into a kiln for heat hardening silver. I have looked through the archives and have some tips. My needs are strictly for hardening at this time but I can foresee enameling as a possibility. My wonder is the economics of buying or building. What I see in the kilns at the sires I have visited is pretty basic technology. My biggest concern is temperature control. (Ventilation is handled already in the shop.)

Is there a minimum practical size for the chamber? Is there a limitation to efficiency based on cubic capacityof the chamber and the amount of pieces being treated?

Is there a best brand? The Durston of KIlns so to speak.

Don Meixner

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Don, there is one design element to consider in a DIY kiln. The length of nichrome wire to use to get the heat range you need.

Many years ago I built a dry-mounting press based upon a DIY article in a photography magazine. The author included a formula to arrive at how much wire to use so one could scale the size of the press larger if one wanted to. I suspect you can get resistance / heating information from wholesalers that make & sell the wire, and possibly on the web.

For the design I followed, the nichrome wire was buried in plaster of Paris or cement (don’t remember which). The problem with that was the heating and cooling contractions of the cement eventually broke the wire rendering the press useless. If you build your own kiln be sure to leave the nichrome wire exposed, generally in channels cut into the firebrink or whatever insulation you use, so you can replace it if necessary.

For temperature control you can use a rheostat and a thermocouple and gauge, which is typical of lower cost kilns. You can add a programming device later if you add casting to your usage.

Hope this helps.

Neil A

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Hi Don,
One of the most important aspect of heat hardening sterling silver is in the second step of low temperature heat treatment. Typically, temperature used is in the range of 550-600F for about an hour. In tests that we conducted at Stuller, we got very good hardness results for an age hardened sterling sample. But when production started to use the suggested time and temperature, the hardness was a good 25% less than what we got during testing. Turned out that in production, the sample was laying on the floor of the kiln, while during testing we had raised the sample about an inch above the floor of kiln on a make shift steel mesh bench! When we measured the actual temperature of the samples (kept on the floor of the kiln) in production, it was a good 75-100F lower than the set point of the kiln (550-600F). Once we raised the samples away from the floor, the hardness was back to where we expected and everybody heaved a sigh of relief!

Bottom line - keep your samples to be age hardened well above the floor of the kiln so it can reach the desired temperature.

Shan

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The largest piece you will put in the kiln should be your guide to the interior size of the kiln.

For the best heat control, do not include a window.

Here is a photo of channels in the walls similar to Neil’s suggestion.

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I just wanted to add to this advice that newer kilns like mine encase the wire element in quartz tubes.
I would assume there is enough room for expansion/contraction of the wire.

Aurora

“If you build your own kiln be sure to leave the nichrome wire exposed, generally in
channels cut into the fire brink or whatever insulation you use, so
you can replace it if necessary.”

Hi Don,
I am a big skinflint and DIY guy and applaud your interest in kiln building…however, I did a little looking on line for various parts…about 30 or more soft firebricks, a standard heating element such as used in the old Satellite kilns, a pyrometer and thermocouple, a rheostat and cord and then some sheet metal for the kiln covering. Adding to that a lot of work in creating a channel in the firebrick for the element and bending all that sheet metal to fit, adding a hinge and latch, etc. I came up with about $250+ in parts and found a kiln on ebay for $475. For the savings of about $200 - 250, I would pass on building, get a manual kiln and spend the time making jewelry instead. Where you might save is in buying a kiln programmer to add to the manual kiln you buy, as they are available pretty cheap on ebay. If you could find an old burnout kiln in need of a new element, that might be a different story…just my opinion…good luck whatever you decide to do! -royjohn

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Once again the metals wizard Shan who payed waaaay more attention in school than most folks, well certainly me, comes through with great info.

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Don…Per our discussion yesterday, my kiln inside dimensions are: 8.5" wide X 5.5" high X 9.0" deep. I use a square of heavy screen to support the piece being treating about in the middle of the available space. This leaves lots of room for circulation around the metal. The controller is from Auber Instruments and I don’t think that I have much more than $180 in the controller and K type thermocouple. I did have to rewire the kiln as the mice had gotten in to it. Hope that helps. Feel free to come by and take a look or even try to heat harden (age harden) a piece…Rob

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Thanks to everyone who has offered a suggestion and advice. I have watched the videos, shopped the web, measured the available space and… I am still pondering.

The boatyard mechanic in me says, “How hard can this be?” and it isn’t hard at all. Spend the money, buy the parts, spend the time assembling. The time spent may be the most expensive part of all. If it takes four hours to build at my shop rate that’s half the cost of a basic kiln and sell a few bracelets and that covers the rest. I am thinking the price isn’t the issue at all. Maybe it’s the “You got this Donnie!” thing and the retired technician is looking for a build.

I’ll let you all know what I do and thanks again for the help.

Don Meixner

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For a kiln on the cheap, take a bucket that matches your desired inside diameter, wrap any tube around the bucket, in a spiral from bottom to top (this will be the groove for the nichrome wire) screw it in place from the inside of bucket.
Find a second bucket for the outside diameter, you want about a 3-4 inch wall. Place the bucket over the inner bucket with the tube. Place a rod at the beginning and end of the tube, through the wall, for electrical connections
Get some fireclay mortar or I’ve heard of people using concrete and perlite, make a thick slurry and pour/pack in in the space between the buckets.
When it is about 80-90 percent dry remove the screws from the inner bucket and remove it.
Carefully remove the tube, you may need to scrape some of the mortar away to get the tube out.
Place the nichrome wire attached to long bolts that go through the wall of the kiln, in the groove. Pinching several spirals of wire between two nuts works well.
Attach electrical leads to the bolts using controller of your choice.
Mine has been going strong for years even with a few cracks in the mortar.
Jim

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Jim…Can you post any pictures? Thanks…Rob

Hey Rob, I’m sorry I didn’t take any pictures when I was building it.
If my description is lacking I’d be happy to give some more details.
Just let me know what needs more clarification.
Jim

That’s OK. Sounded cool, I can imagine it. Thanks…Rob

@Jim_Doherty1

That’s brilliant!
How did you form the lid?

I’m guessing you remove the lid, instead of including hardware? Now that’s when the perlite would be effective.

I’ll add a bit more detail, I kind of rushed the first post as I didn’t want to write a book.
The inner bucket should be straight walled, the tube should be an ascending spiral from bottom to top around the inner bucket attached by screws from the inside into the tube so you can remove them to get the inner bucket out when dry, a bit of oil to act as a release on the outside of the bucket helps to get it out.
At the beginning and end of the tube place a rod or dowel through both containers to make a channel for (I used threaded rod) the electrical connections. Make an indentation(a blob of clay or a small piece of wood so the nuts pinching the wire will sit in the fireclay and not intrude into the chamber. These are at the ends of the tube where the dowel rod goes through.
Reading back through my post I noticed a mistake I believe it was cement and not concrete
with perlite, I used fireclay so I can’t really comment on the effectiveness I just remember reading that someone used it.
For the outer “bucket” I made a straight walled container from sheet metal, some screws through the outer wall will give the fireclay mix something to grip keeping the mix adhered to the wall of the outer container.
The base and lid were made in the same fashion a 3"-4" tall sheet metal ring filled completely with fireclay mix, attach handles on the lid to lift it and legs to the base (just flip it over when dry)
So you have 3 pieces, a base, the center with the nichrome wire that sits on the base and a lid that just sits on top.
Pass threaded rod through the wall in the channels made by the rods or dowels, use 2 nuts to pinch the nichrome wire, attach electrical connections to the threaded rod (kind of a safety issue as the exposed threaded rods and clips are hot electrically) but I use mine outside in an enclosed area so no kids or pets can get to it.
I use battery clips (like alligator clips but larger) not jumper cables, to clip onto the threaded rods and plug that into my controller.
I only use mine for burning out wax and preheating metal to cast so very simple.
If you wanted to add a thermocouple you would need to add an additional rod through the wall to give you an extra channel or make another piece with a channel (that could be added or removed as needed) and have a 4 piece unit.
I used about 18ga nichrome wire it was rated to 2000 degrees, stretched the coil by guestamation, if the coils are too close the wire will burn and separate. about a 1/4" between coils seems fine, just grab the ends and pull till it looks right.
Really not difficult to build and I have about 100$ into it.
I wrote this just to give an idea of what you can do, these are not instructions and if you are not comfortable and confident of your ability to take these ideas to fruition then I suggest you buy a premade unit for safety’s sake. If anything needs a more detailed description let me know.
Jim

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Jim…Thanks for the extra detail. I have a perfectly good kiln, but I am a fan of DIY solutions…Rob

@Jim_Doherty1

I wonder if forming the wall and bottom in one piece would increase heat retention? It could be done by raising the the inner bucket, like making a pot.

I don’t see why it wouldn’t work, that is the great thing about making your own, do what works for you.
I made mine in 3 sections as my inner chamber is very small and I wanted the option of being able to lift the heating chamber off the base instead of having to use tongs to reach down and retrieve a flask or crucible.
Jim

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