Top loading Kiln question

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Hello.

I recently acquired a small top loading kiln with an element in the lid. The kiln is 110v, with a heating cavity that is 9 inches deep (two bricks tall,) and 17.5 inches wide (simple dial control, but a thermocouple and analog temperature display were included in an accessory box.) I was thinking it could be used for heat treating with a few improvements, but wanted to see if anyone had some wisdom to share before I get too involved.

I was thinking about installing a PID, cutting a hole in the side of the unit (basically removing a brick, and using that as a door/plug for inserting and removing blades,) and either lining the bottom of the kiln with a layer of firebrick or filling the first 4-5 inches of the kiln with vermiculite.

However, I am not sure if having the elements on top (above the knives) is a great idea. All the commercial ovens have elements on the side, not on the top.

I was all set to order an evenheat oven, but balked because they wanted $350.00 to ship it to my location. I thought I would attempt to build one or modify a kiln instead.

Thanks.
 
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Where are you locate? 350.00 for shipping sounds outrageous. Even on a pallet I would expect $150 shipping.
 
I would stand it upright and make racks to hold your blades. Sounds like it was a pottery kiln...make sure it will reach the temperature you need. Removing a brick will probably interfer with the heating design of the kiln interior.
 
Thanks. I was planning on standing it upright, but since the element is in the lid, and not the walls, I thought that I could make a door in the side of the unit by removing one of the bricks, or cutting a hole (the walls are only two bricks high (9 inches.) It also looks like the bricks that make up the walls are cemented together. One of the things I am unsure about is how will an element directly above the knives (since the element is in the lid) affect them (instead of having an element on either side.) Another option would be to remove the element from the lid, and make some new walls inside the kiln, changing the internal heating area into a channel instead of an octagon. Then I could install elements in the walls, and maybe change it to 220v (I have an unused 220v 20amp circuit in my garage that used to be connected to a baseboard heater.)

The kiln looks a little like the one on this page:http://www.sheffield-pottery.com/SKUTT-KILN-GLASS-FUSING-KILN-HOTSTART-PRO-p/skhsp.htm
(But without the fancy electronics.)
 
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One or two thoughts.

The dial control is less than ideal. It can work, but takes quite a lot of fiddling to get the temperature right and the last few degrees as you approach final temperature are painfully slow. PID is better.

Installing a PID is pretty easy if you feel comfortable with that sort of thing, but probably not wise if you don't.

I'd agree with the "upright" suggestion.

I'm pretty sure that, within the limits of what you have, you want the blades as far away from the elements as you can, to minimize the heat differential across the blade. Raising the floor does not seem like a good idea to me. Raising the floor with hard firebrick (I'm not sure what you can get, but here it's easier to find than IFB, being used for firebacks), that will be slow to heat up and will therefore increase the temperature difference across the blade as the kiln heats up, seems like a bad idea.

I'd fit the PID, make a rack to hold the blades upright, probably look for a piece of stainless steel (304, 316, etc; just "normal" stainless, not blade steel), or ceramic paper, to lay on the top of the support pins of the rack and shield the blade from direct radiated heat. Then I'd see how it worked as a top-loader before tearing into the bricks.

I always try to only change one thing at a time. If you make changes to the control at the same time as changes to the structure you might get lucky and find it works great. If not, it's much harder to work out which of your changes made things better and which made things worse.

I'd also suggest you don't try to temper in it, at least until you have got Austenitizing properly sorted. If you are going to temper in it, burying the blades in dry sand will help even the temperature out; Upright shouldn't be necessary for this, so you can have them laying down with a decent thickness of sand above and below.

I've built half a dozen HT ovens (I'm in the UK, where the price difference between a complete HT oven and the parts to build one seems to be much greater than in the US). For each one, I've probably spent as much or more on the shipping of parts piecemeal than it would have cost to ship a complete oven, despite being able to get the Insulating Fire Bricks locally and avoiding the extortionate shipping cost they'd otherwise incur.
 
Thanks for the input, I greatly appreciate it! It is ironic that I can find hard firebrick at our local hardware store, but no one in town carries soft firebrick. Knowing how fun it is to get things shipped here, I suspect any soft firebricks I order would probably arrive as a bag of dust... (I ordered a cast iron frying pan a few years ago and they managed to break that.) :)

My thought about raising the floor was to try and make the kiln more efficient by having a smaller cavity to heat, but I understand what you are saying about the potential for heat differentials the closer the blades are to the source.

The kiln did come with several octagon shaped "racks" or shelves of hard firebrick material (pretty dense stuff,) as well as some pillars (I believe the previous owner said it was used for making glass jewelry.) Should I use one of these, or just a piece of metal as you described in order to shield the blades? I imagine it will take a bit longer for the racks to get up to temperature (they are fairly heavy.)

I created an imgur account and have uploaded some images of the kiln to help illustrate what I have to work with:

http://imgur.com/HmEIwx9
http://imgur.com/A7TOgMV
http://imgur.com/7LO4DQI
 
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I had a feeling it was an enamel/glass kiln. It won't work well at all for blades. It is made to get frit ( powdered and chunks of glass) to the melting/fusion point and for firing them as enamel. This takes place at around 1000F, and the ovens rarely need to get above 1200F. For austenitizing knives need an oven that can easily reach 1500F or a bit more and hold it well. I know the thermometer and specs will say something like "Reaches 1650F", but trust me...it won't do it. Save all your effort and money and build/buy a real knife oven.

Now, this oven may work fine as a tempering oven with the addition of a PID control. Just because it won't run hot does not mean it won't run warm :) . Forget putting the blades in a pan of sand, just pre-heat the oven to 400F for 30 minutes before putting in the blades. ( or the desired temper). The newer digital control glass ovens have the built in program to temper/anneal the glass and will work perfect as a tempering oven.
 
Thanks, Stacey.

There is a plate on the side of the oven that states that this is a 1725 watt kiln, max temp of 2100F. It is labeled "crucible kiln," which I believe is a brand name from Seattle. It appears that there are about 30 firebricks (16 in the walls, and the rest make up the top and bottom.) I guess my next step is to get a PID and see what temperature it is capable of, and then decide where to go from there. I was thinking as a last resort I could dismantle the kiln and reuse the bricks to build a new knife oven, as shipping 30 soft firebricks here costs about as much or more than what I paid for the kiln (not to mention that they would probably arrive damaged...)

Here is a link to the plate on the side of the kiln:
http://imgur.com/TXV7biD
 
The very first thing to do is to put the thermocouple and analog display in, set it to maximum and see how hot it actually gets.

If it gets hotter than you need, you can decide what you feel is worthwhile in terms of modifications. If it doesn't reach the temperature you need, the mods are unlikely to be worthwhile and you can move on.
 
OK, a crucible kiln is normally used to melt glass for glass blowing and glass art. There are different types, but all I have seen are for glass work. Yours is a top element style, and may be for making fused glad art work and things like bowls and vases. The kiln probably isn't meant to be opened while hot. The materials are put in the cold kiln in a shaped crucible or on a tray, and the program is run. The glass melts, fuses, and slowly cools over as many as 24 hours. The cold oven is then opened and the crucible or art piece is removed, or more glass is added and another cycle is run.

The top rated temp on large door kilns is mostly hyperbole ( glass, pottery, jewelry). Many/most jewelers burnout ovens are all rated at 2050-2100F...but you will never see it get beyond 1900F. Even then it can take a couple hours to get up to 1900F, and if you open the door to put it will drop to 1700F and take an hour to get back up.

If you already own the kiln, try doing a 1084 blade in it as is. Just put the blade in once the oven is at 1475F and close the lid. When it is back to 1475F, switch it off, open the lid, remove the blade while wearing a kiln gauntlet and using long kiln tongs. If all goes well, revisit your idea about modifying the side wall and adding a door. You will burn up the element if you open and close the lid too often while hot. I would not add the PID control until you decide it is worth the trouble. Manual control of the analog dial will be fine for most all carbon steels. These type of ovens don't have wild swings in temps.
 
Thanks for the input. I do not have anything to harden right now, but just spoke to Kelly Cupples and placed an order for some 1080 and 1075. In the meantime I will stick the provided thermocouple in and see how hot it gets (and how long it takes to get there.)
 
Here are the results of test firing the kiln:

In took 10 minutes to get to 500 degrees.

It took 21 minutes to get to 800 degrees (from start.)

It took 40 minutes to get to 1000 degrees (from start.)

It took 125 minutes to get to 1500 degrees (from start.)

It took 240 minutes to get to 1725 degrees (from start.)

I turned the kiln off after 4 hours as I just did not see a point in continuing. The kiln was pretty hot on the outside by that point, and I am guessing that the heat loss was attributing to the slow ramp up times (at least after it reached 1500 degrees.) The analog display was also probably off a bit, as from a cold start in my garage at 55 degrees it was reading about 90 degrees.

At this point I am wondering what I should do - I could:

1. Try and use/modify the kiln, and add some interior walls (smaller heating area) with new elements (probably rewire it for 220v.)
2. Break the kiln down and try to recycle the bricks to build an oven (this may be difficult as they look like they are cemented together; maybe use a table saw to cut them up.)
3. Do nothing and either sell the kiln or use it for tempering only (install a PID,) and build an oven from parts.
4. Decide to eat the $350.00 shipping cost and order an Evenheat.

I am leaning towards option 2, but am open to suggestions. I have just about all the tools needed to build a new frame (no welder, but I could bolt or rivet it.) I have no qualms about the electrical work, but may need some assistance when it comes to getting the right elements (I am thinking about making something similar to the Evenheat KO 22.4, but a bit longer - like 24 inches.)

Thoughts?
 
I would go with #3 and #4.

If you can sell the kiln, do so. If not, just add a PID and use it exactly as is for tempering. The temperature will be so low that you will not have a problem with the top loading. Just pre-heat it and set the blades on the kiln floor. I would NOT alter it beyond PID control. The PID control will only require a thermocouple and a PID. The wiring changes/connections are very simple.

I HIGHLY recommend you buy an 18" deep knifemaking kiln and just consider the shipping as a cost that you need to spend.
 
I agree.. Your going to be spinning your wheels with that kiln. When you get a proper kiln you open the doors to a whole new world of heat treating options. It just better all around to have it. If you just can't afford the extra $350 which I agree is a lot but understandable due to your location. Then by all means do what you got to do.
 
Stacy and Mark beat me to it.

Building an HT Kiln is not trivial. Nor is it cheap. It sounds like you'll be eating shipping costs on whatever parts you need to get shipped in anyway if you go for a self-build.

Paying the shipping on a complete kiln is a cry once deal. From here, it looks like the best option.

Trying to sell the oven complete, or keeping it for tempering, at least retains the value in what you have.

If you are seiously contemplating option 2, make sure you have a design that will work with the bricks you have. Using them will not be like using new materials. I find it's fairly easy to get 9" x 4 1/2" x 3" IFBs to span a 7" gap for the roof. I don't know how they've been cut for the octagon on yours, but I suspect you'll find it less easy than with full bricks. I used 30 full bricks in the last one I built; 22 1/2"L x 7"W x 6"H internally, and I think I could probably have got away with "only" 29. Trying to work it out with cut bricks would give me migraine.

I wouldn't really recommend it because it means butchering something with value, but if you are seriously considering option 1, grooving the inside of the octagon for a suitable 220V element might be achievable with a router. The element would go in a flat-rate box, as would a PID controller, a thermocouple and an SSR, so shouldn't be too bad on shipping cost. I'd strongly recommend a ramp/soak PID controller. The one I use is the Omega CN7823, but the Auber Instruments ramp/soak controller seems popular your side of the pond and there are probably more users whose brains you can pick.

Details on the Omega: http://www.omega.com/pptst/CN7800.html
 
If you are capable of building one then I would go that route. I would find it very hard to believe that there aren't any soft firebricks in Alaska that could be shipped easily or even picked up... Pottery suppliers?

Kanthal wire wouldn't be much for shipping from the lower 48 and possibly a pottery supplier that has firebricks may even have the wire too, for building and re-fitting kilns.
 
Thanks for all the great reply's. There are a lot of good points being made, and I appreciate them all.

My comments about the shipping cost are mostly based on principal than anything else. I can afford to get the heat treat oven shipped here, but I get tired of the shipping rates for Alaska. The ironic thing is that the packages arrive here via the exact same means independent of what they charge for it. 99% is via airplane. So when we pay for something to be shipped first class or parcel post it goes out on the (usually) same plane and (usually) gets there at the same time. A lot of times the packages get left on the back dock of the post office to create the illusion that package A that was priority mail got here sooner than package B that was sent parcel post... Lately I am beginning to think they are only getting mail twice a week now, due to when the mail is arriving.

UPS is no different (2nd day air, ups ground - both have to get here via the same plane.)

But something can be said for just buying it so I can get on with making knives instead of shipping them out to get treated & waiting for them to return...

I have a lead on a person in Fairbanks (700 miles North of me) that may be able to provide some soft firebrick (I still have to run the risk of them getting broken on the way here, but the shipping costs should be less.) No one in Juneau has any (no pottery supply, etc. here.) There are some wood stove vendors, but they only have hard firebrick. The University branch here has an outdoor kiln - but they get theirs barged up from Washington.

Here is an idea I will throw out there: How about making an oven from a lattice of Kanthal wire, and wrapping it in several inches of refractory blanket? the elements could be attached to the lattice (probably need insulated from it to keep from shorting out,) but it would be light weight and resistant to the heat levels we are using. Kind of like the opposite of what people are doing with firebrick & angle iron. A hollow wire frame that holds Kaowool (or its equivalent), or just an internal wire structure that is wrapped in Kaowool...
 
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That would be fine if all you cared about was heating something up....like bending glass or plastic, or such. Unfortunately, we need a lot more control and accuracy in a knife kiln. You need a thermal mass for that. The oven walls and floor provide that. There are ovens with cast linings with the coils imbedded in the cast refractory. These have very slow and even heat control. But, just insulating them from the outside with a refractory blanket would mean that the only heat would be radiated heat...and that would be very bad as far as control.
 
Good point. I had not thought of that.

The K23 firebrick (9" x 4-1/2" x 3") in Fairbanks is slightly tapered (2 & 7/8" thick on one end and 3" thick on the other.) Cost is $3/brick plus shipping (so probably $4-$5/brick to get them shipped here.) I am thinking that these will work with a little planning.
 
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