Kiln Max Temperature

[yinjuehua]

I recently acquired an old manual Sno Industries Kiln with a sitter, operating at 110V. The seller mentioned that this kiln can only fire low-fire clay, and so far, everything has been fine using low-fire clay and glazes. I couldn't find any information about this kiln online. It's quite small, with an internal size of approximately 11 inches in diameter and 13 inches in height, and the firebrick walls are 2.5 inches thick. I have seen many commercial 110V kilns that can reach cone 6 or higher, but I would really like to try mid-fire clay and glazes. I want to know what risks are involved in doing so. Could the elements burn out? What determines the maximum temperature a kiln can reach? Thanks!

[Bill Kielb]

The maximum temperature a.kiln can reach predominately depends upon the power supplied to it and the shell losses. L&L has some great measurements. Looks like to be safe ~3-1/2watts per internal square inch surface gets you to cone 10 rating. Firing elements hotter makes them wear out much sooner.

[yinjuehua]

Thank you for the information! So I can safely try to fire cone 6? I'm not very concerned about the lifespan of the elements. I plan to buy a PID controller and make a new set of elements myself next year.

17 minutes ago, Bill Kielb said:

The maximum temperature a.kiln can reach predominately depends upon the power supplied to it and the shell losses. L&L has some great measurements. Looks like to be safe ~3-1/2watts per internal square inch surface gets you to cone 10 rating. Firing elements hotter makes them wear out much sooner.

 

[Bill Kielb]

6 hours ago, yinjuehua said:

So I can safely try to fire cone 6? I'

Since you may wind your own elements, you may want to calculate first and see what you have watts/ sq in. Presently.  Being able to go 100 degrees per hour in the last 200 degrees of the firing is sort of a sweet spot for cone predictability and can usually wear down to 60 degrees per hour in final 200 degrees, but usually not a whole bunch lower. Gives you an idea of a target final speed I think in your test fire. It not only has to make it but make it with reasonable ending speed.

Edited May 25 by Bill Kielb

[Dick White]

Sno Industries has been out of business for a looooong time. If you look at the manufacturer's label and the address does not include a zipcode, that is a sign that it was manufactured before1963 (that's when the Post Office started using zipcodes).  As Bill points out, the maximum temperature is a function of the power (in watts) of the kiln elements vs. radiant heat losses through the shell, through both the side walls and the top and bottom. The bricks are not perfect insulators, and the amount of heat lost through them increases with time and temperature, as the brick becomes saturated with heat and insulates less. 120V kilns typically generate 1800 -1900 watts (a big hair dryer) as any more than that will blow the typical household circuit breaker. Current cone 6 rated commercial 120V kilns are all smaller than your kiln and have 3" brick, so the available wattage of heat from the elements disperses less through the walls of the better insulated, smaller kiln, and thus can reach a higher maximum temperature before stalling when radiant heat lost equals heat input.

I have done some of my own rough calculations on the entire Skutt and L&L lines using rated watts of heat, external shell dimensions, and manufacturer's stated maximum cone. The typical cone 10 kilns generally had at least 3½ watts per square inch of external surface area. Kilns rated to cone 8 (but only good to cone 6 in the long run) had about 3 watts per square inch. Kilns rated to cone 6 (which would go through elements quickly if consistently fired to cone 6) had between 2½ and 3 watts per square inch. And kilns rated for less than cone 6 had less than 2½ watts per square inch. Your kiln is looking like about 2 watts per square inch of external surface area, not likely to reach cone 6 IMO.

As Bill notes, you can wind your own elements to any specification you want to get more heat, but more heat is more amperage of electrical service, and your house wiring needs to be able to support it. An option, if you are going to rewire the kiln - convert it to 240V. You will need to do a bunch of calculations to match your new element windings to the kiln wiring and house circuit, but it can be done.

[Min]

23 hours ago, yinjuehua said:

and make a new set of elements myself next year.

I would suggest pricing out what the wire would cost you compared to getting them made from Euclids. We did wind our own elements for quite a few years, using 12 guage wire we wound them onto a mandril on a metal lathe. It's easier said than done. Came a point in time when the price difference was less than $100- for a set for a 10 cubic foot kiln and no longer worth it. Euclids can also help with figuring out the calculations if you supply them with kiln size, electrical available, required temp reached etc.

[yinjuehua]

Thank you all very much for providing all the information; it has been incredibly helpful to me!

 

[Kelly in AK]

I noticed kiln manufacturers switching from 2-1/2” brick to 3” brick as a standard, or at least more commonly, over the years. Given that shell losses are one of the variables at play, pure curiosity makes me wonder: how much could be gained by wrapping an existing kiln in fiber blanket, or a second layer of soft brick?

[Babs]

50 minutes ago, Kelly in AK said:

I noticed kiln manufacturers switching from 2-1/2” brick to 3” brick as a standard, or at least more commonly, over the years. Given that shell losses are one of the variables at play, pure curiosity makes me wonder: how much could be gained by wrapping an existing kiln in fiber blanket, or a second layer of soft brick?

Careful, Wrapping kiln in fibre blanket, or top bit melted the metal sliding thingie over spyhole in old kiln i had and, handling ceramic fibre blanket is a bit hazardous.

[Dick White]

Joining @Babs in recommending against wrapping a kiln in an additional layer of fiber. The fiber will trap some escaping moisture against the metal shell, resulting in more rapid rusting of the metal parts. Kilns that have a layer of fiber around the bricks have the fiber inside the metal shell, directly against the brick.

[Bill Kielb]

2 hours ago, Kelly in AK said:

how much could be gained by wrapping an existing kiln in fiber blanket, or a second layer of soft brick?

It’s really no different than adding insulation to your house. Adding double reduces losses to ~ half. R value of materials are basically additive for homes, kilns are similar. How it’s done is key so if just building a kiln no worries. If retrofitting then removing the steel shell easiest way. The lid is also an issue as over heating the already banded bricks can cause larger deflections and lid failure. Easiest improvement for the effort I have seen is Add 2” rigid to the interior of top and bottom where maximum heat loss exists anyway. But to answer your question, more insulation, less power to make cone, but also slower to cool down.

Edited May 29 by Bill Kielb

[neilestrick]

Don't wrap a kiln in fiber. You'll destroy the metal that way, and fiber is not friendly to your lungs. If a kiln is working properly, then there's no need for additional insulation. Different kilns are made for different applications. If your kiln doesn't get hot enough for what you're doing, you need a different kiln or you need to swap out the elements for ones that will produce more heat. However comes with other potential issues such as needing to upgrade all the kiln wiring and service wiring to accommodate the increased amperage. If you aren't familiar with electrical circuits then you shouldn't attempt any changes like that.

Top loading kilns are a series of compromises. They can easily build kilns with 4.5" thick walls, but that greatly increases the weight of the kiln, making it more difficult to move and increasing shipping costs. Thicker walls also slow down cooling times which give you less control over the firing, and it requires more bricks which means higher costs. 3" bricks are a simple change from 2.5" bricks, though. The weight and cost increase isn't enough to have a big effect, and the increased efficiency is worthwhile. We've had this conversation several times over the years, but I'm not convinced that adding fiber insulation behind the bricks on round kilns is worth the effort. Firing costs are pretty darn low when you break it down to cost per pot, so the cost benefit is minimal and there are downsides when it comes to repairs.