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I am using a recipe for kiln wash that is 50% alumina hydrate, 25% EPK, and 25% calcined EPK about the thickness whole milk. I usually apply 2 coats by brush or 3 by roller. It seem like after 2 or 3 glaze firings any runs do not release from the shelf. I just had a cylinder that the glaze ran pretty badly and in trying to remove it from the shelf most of the bottom of the cylinder broke off and I had to use a hammer and chisel to get the pieces off. 

Do I need to strip and recoat the shelves after every glaze firing ( I hope not) or am I doing something wrong?

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No, you shouldn't have to strip and re-coat after every firing. Are you 100% sure that all the ingredients are what they're supposed to be? 

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Min,

I am not 100% sure. The EPK is use all of the time in making glaze so I am pretty sure it is okay. The alumina hydrate I am less sure of. I have had it for several years and only use it for kiln wash. I will replace it and try again. 

Is the a general rule on how often the strip and recoat the shelves?

The glaze that ran was a commercial cone 5 and I am firing at a hard cone 6 because of vitrification problems with my clay. I assume it was over fired thus causing it to run, however, if the kiln wash was doing it's job shouldn't cylinder easily come off of the shelf?

Thank you for the help.

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Some glazes fume or spit more than others, a super rough ball park figure would be in the 100 glaze firings range. The alumina hydrate won’t spoil over time so if you are certain it is what your container of it says it is it will be fine. Glaze drips should pop off the wash but if you have a pool of glaze drip onto the shelf then it will take some grinding to get it all off.

If you are really set on using that glaze and over firing it there are a couple things you could try. First off put a waster biscuit/wafer of clay under the pots with this glaze on it and apply wash heavily to the waster. You can reuse the waster slabs and if you get a bad glaze run just toss the waster instead of grinding shelves. Secondly, try spritzing the outside of the pots with water, lower few inches of the pot, wait 5 minutes for the water to soak into the bisque then dip your pot. The damped area will take up less glaze. If you are brushing the glaze on try a thinner application near the bottom of the outside of the pot. 

Another thing you could try is to alter the glaze. If it’s a dry glaze you buy try adding some EPK to it. 100 grams of dry base glaze plus 5 grams of EPK, dip a test tile to the same thickness of glaze as you would use on your pots. Add another 5 grams of EPk and dip another test tile. If you buy liquid glaze then weigh out 100 grams of glaze and try 3 grams of EPK, dip your tile, add another 3 grams EPK and repeat up to 9 grams of EPK. Draw a line on the bisque with an underglaze pencil or a line of iron oxide to show where the glaze comes to on the test tile, draw another line 1/4" or so below that, use this as a gauge to see how far the glaze runs.  (EPK contains alumina, raising the alumina content of the glaze will stiffen up the glaze therefore run less)

Going forward I would either find a clay body that is vitrified at ^5 to match your glaze or change your glaze like OldLady said.

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I think that Alice and Madeline are both right about how to fix this problem.

Regarding the drip situation on the kiln wash, your wash recipe is one of the better ones. If you apply it only with a super thin coat though, and your glaze runs a lot, the glaze will still soak through. Kiln was will pop off if the drip is small or there's just a bit of plucking. If you have a massive run like you describe, it will minimize the shelf damage, but there will still be grinding in your future.

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Thicker layer of wash on your shelves. If there's enough of a run to get through the wash to your shelves it's gonna stick as if there wasn't any wash on there to begin with. Add 1-2% g200 to your wash; allows you to build a very thick layer. I used to have about 1/2" of wash on my shelves...then j got advancers. 

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After scraping and sanding my kiln shelves, which was a 4 hour aerobic excercise, I tried the kiln wash recipe 50% Aluminum hydrate, 25% EPK, 25% Glomax, then added 1.5% G-200.  Applied the wash in a milk like consistency.   While the wash was drying I noticed cracking.  I smoothed it over and it lay flat.      After the firing, the wash crazed but not peeling.  So disappointed!!!!  What to do next!

Attached picture. 

IMG_2529.jpg.2f83fc7648c52ad94a0fb05b307ab29c.jpg

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Why add the G-200???

I mix mine like heavy cream and roll one coat-let it dry in sun then apply 2nd coast

Could you explain your application process so I can diagnose why this happened . Also do not use the g-200-no need for it.

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It's too much kaolin. That's the only thing in the recipe that shrinks a lot which is causing the cracking. You only need about 15%. I'd also cut the calcined down to about 15%, and use silica for the remaining %.

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I used the recipe listed by Dave K. (advanced member), then added G-200 that hitchmiss (advanced member) suggested.

 After scraping I sponged off excess dust, etc. - not a lot of water.  I mixed up the ingredients and brushed two coats on the cleaned shelves.  They sat out in the sun for half day, then dried in the studio another day.  I then used them in a cone 6 glaze firing.

So now that the shelves are all crazed and peeling I guess I'll have to scrap them again.

Neil - What is the recipe you use?

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i think both dave and liz have learned from their experiences.

the recipe dave used is the correct one but he expected the piece stuck to the shelf by too much runny glaze to survive.  the kiln wash is designed to protect the SHELF  not the pot.  if you make a pot and use the wrong glaze, you lose the pot.

liz altered a recipe that was perfectly ok the way it was.  why alter it at all?  half an inch of kiln wash is not necessary if you glaze and fire the correct cone number on both the pot and the glaze.

 

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