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Sputty

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My understanding is that cones measure heatwork penetration into the ceramic body which, as you describe, is a matrix of rate of temperature increase vs. absolute temperature, while the glaze, being quite thin and on the surface, will melt to it's proper maturity at a modest range of absolute temperature. Typically, you would match the glaze melting temperature to cone needed for the body.

 

You can fire a load to cone 10 but never exceed 2200F by holding it there for 12 hours. This is common in woodfirings. What I don't know is whether you could do this in an electric kiln (rate of increase at the end 100F/hour, hold at 2200F for 12 hours) and adequately melt a glaze that is nominally rated cone 10 (i.e., an expected glaze melting temperature 100+ degrees hotter)? In the woodfiring, the ash in the kiln atmosphere will flux the glazes, inducing melt at lower absolute temperatures than would be expected in the clean oxidation atmosphere of the electric kiln. (Regarding your hypothetical 1650F temperature, a calculator I have from Orton Ceramics suggests that 1650F will max out to cone 04 after an 80 hour hold, never getting beyond 04 despite proposing absurdly ridiculous hold times.)

 

A further complication is that the glaze issue is not limited to just its own melting. There is also interaction between the glaze and body where some of the oxides in the body begin to melt and fuse with the melting glaze. It is not a discrete boundary between glaze and ceramic body.

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If you look at a manufacturer's cone chart, you will find end temperatures that vary depending on the rate of temperature rise over the last 200 degrees or so of the firing; so, a cone 6 cone will have a different end temperature if the heat rate is 60 degrees an hour for the last 200 degrees vs 150 degrees and hour for the last 200 degrees. That would explain your end temperature variance. So long as the cone tips properly, your glaze is getting the heat work needed to melt.

 

The last variable is your kiln brick. Thicker 3" insulated fire brick is a better insulator and holds heat better than 2 1/2".

 

As for eutectics, it is not only a matter of hitting the melting point temperature; you also need to hold the heat to allow for a smooth melt. You can put a pizza in the over and bring it to the baking point (say 400 degrees); take it out, it may be uncooked. Leave it in for 18 minutes, and it has had time to absorb the heat and cook through. Same for pots and glazes.

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Yes time and temperature are to some degree substitutes for each other, but this principle is not absolute.  Cones are just the mile markers along the side of the highway, not an indication of what is happening on the highway itself.

 

During the firing, as the temperature rises, more and more individual materials and groupings of materials (eutectics!) start to melt.  As the melting process snowballs the whole glaze surface slowly converts from a bunch of dry powders into a sticky, viscous liquid.  The greater the quantity of liquid, the more materials get pulled into that melt solution.  Eventually, all materials are bathing in the molten mass.  Having materials IN the melt rather than standing alone isolated with only air around them increases the speed at which they break down, all else equal.  Part of this is likely eutectic and the other part is the efficiency of heat transfer being much higher in a solution than in air. 

 

However, just because all materials are part of a mostly-molten mass does not mean that the glaze has matured.  Some materials in the melt are more refractory than others, and their eutectics to liberate their key oxides are likewise higher.  They DO require that a certain minimum temperature be achieved and held for a certain amount of time in order for the glaze to properly mature.   Boron and zinc and many frits melt early and often, but materials which need to liberate calcium and magnesium to involve it in the melt - never mind silica and alumina - are often much more refractory. 

 

And particle size makes a difference for the time to maturity.  Silica is silica and always melts at the same temperature, but fine 400 mesh silica melts much quicker than 40 mesh grains of sand. 

 

 Anyone who has struggled to get a wood fired kiln to temperature and consoled themselves with the idea that "its OK, we will just soak those cones down" knows that for the most part this does not work.  Even after hours of "soaking" at a sub-optimal temperature, glaze may be firmly stuck on the pots, but those cone 10 glazes will look under-fired and immature (eg, matte looking surfaces on a glaze which should be glossy) no matter how many hours and how much ash and other volatiles you hammer them with, if you only ever get cone 8 down and cone 9 soft.  And at that point, as you are staring glumly at the dull glaze surfaces, you aren't even worrying about whether or not the body got matured.

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That explains it all for me!

I recently did a ^6 glaze firing in my very old Cress B23H. After running all night (My controller said it ran for 14.4 hrs, reached 2114 degrees and shut down because it temp would not advance anymore) and after cooling down, the cones on the bottom shelf were hardly affected while the cones on the top shelf were greatly affected The 5 cone was bent all the way over but not melted flat; the 6 cone was slight bent; the 7 cone was not affected.My presumption here is that since the kiln ran for such a long time at the ^1 temp, it was the heat work that melted the glazes. It was a kiln full of 22 small bowls and vases/fragrance diffusers. I was doing a test of Amaco Potters Choice layered glazes, and the results were what I expected in some cases and surprises in others. I was somewhat disappointed that the whole kiln did not make it to ^6, but, considering that it is a very old kiln and had tired elements, I was not surprised, since it was also a test to see if it would reach ^6 before I sold it. I've had it on the market as a bisque kiln but have not had any luck selling it, so what I'll probably do is dismantle it, save the really good bricks, use the elements to make my own pin supports of varying sizes, recycle what I can and put my ^10 kiln into service.  :) 

JohnnyK

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The clay/ glaze interface has a distinct structural bridging of materials: clay is pulled into the glaze and vice versa.

http://community.ceramicartsdaily.org/gallery/image/8291-clayglaze-interface/

 

Several universities around the world have tested " heat work" on kaolin clay bodies. The most striking was from a university in china that stated: once the surface of the body hits the temperature designated, it can take two to three times longer to penetrate to the midline of the clay.

 

An area where I believe modern pottery has taken a step backwards; in that firing programs tend to focus on glaze maturity only. But hey, I am a clay guy...what did you expect me to post?

 

Nerd

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Heat work only goes so far. You still have to have some temperature to get the melt started. I once tested all of my cone 6 glazes to cone 4 with a hold to get to cone 6. Some of them looked fine, but some looked very under fired. They just didn't get hot enough to get the melt started. Those with frits did fine, because the frits melt at very low temps and get the glaze melt started, but those without frits didn't do nearly as well.

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