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adjusting already mixed glazes


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#1 Stephen

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Posted 12 April 2013 - 09:44 AM

I have recently changed clays and have 5-6 gallons of mostly clear glazes that doesn't fit my new clay. I'm thinking about adding 10% silica as an additive and test to see if it will clear up the crazing issues and would appreciate input on doing this. If the test seem to accomplish my goal and give me a good fit should I feel comfortable using up this stock? Anyone here make adjustments like this with bad long term results?

#2 trina

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Posted 12 April 2013 - 10:51 AM

I have recently changed clays and have 5-6 gallons of mostly clear glazes that doesn't fit my new clay. I'm thinking about adding 10% silica as an additive and test to see if it will clear up the crazing issues and would appreciate input on doing this. If the test seem to accomplish my goal and give me a good fit should I feel comfortable using up this stock? Anyone here make adjustments like this with bad long term results?


Hi there, I have found the problem with trying to correct ready mixed glazes it that you don't really know what is in them to start with. That makes correcting hard. Try the silica, but you might be better off testing some completely new glazes. When I was forced to change clay bodies when my supplier went out of business, i had the same problem and still have about 40kgs dry transparent that will stay like it is until i get a body to fit it. That seems way easier to me. T

#3 neilestrick

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Posted 12 April 2013 - 11:02 AM

Run some tests on small pint batches so you don't ruin the big batch. Approximate how much dry material is in your 5 gallon bucket, probably around 9,000 t0 10,000 grams depending on the recipe, and calculate that down to a pint. Better to estimate low, so you don't add too much of anything. Start with 3%, 6% and 10% additions of silica to fix the crazing. Once you've figured out how many grams of silica you need for the pint, convert it to the 5 gallon bucket amount. Remember to screen the big batch after adding the silica.
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#4 Stephen

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Posted 12 April 2013 - 11:15 AM


I have recently changed clays and have 5-6 gallons of mostly clear glazes that doesn't fit my new clay. I'm thinking about adding 10% silica as an additive and test to see if it will clear up the crazing issues and would appreciate input on doing this. If the test seem to accomplish my goal and give me a good fit should I feel comfortable using up this stock? Anyone here make adjustments like this with bad long term results?


Hi there, I have found the problem with trying to correct ready mixed glazes it that you don't really know what is in them to start with. That makes correcting hard. Try the silica, but you might be better off testing some completely new glazes. When I was forced to change clay bodies when my supplier went out of business, i had the same problem and still have about 40kgs dry transparent that will stay like it is until i get a body to fit it. That seems way easier to me. T



Hi Trina, thanks for the input. Actually I mix my own glazes so I do know what in and even pretty much what's been used to a reasonable degree but when I run the old formula with the additive I'm at 110% and that puts all the chemistry out of whack. I am adjusting new batches to fit properly but these old ones are going to have a screwy alumina ratio if I just add Silica and I don't really want to try to start adjusting everything else of as I think its going to get goofy quickly.

I figured/hoped with just plopping the 10% in that I will get more matt and of course hopefully get the fit better to halt the crazing. I guess my first concern is lowering the melt too much as they are cone 6 and I want to take them to cone 6 after the adjustment but I'm sure a host of other issues might arise as well. Hate to toss so much otherwise fine glaze but maybe that's the easiest route. Could keep it around like you suggest for another go with something else down the road but its kinda taking up space I would rather have in production.

Again though thanks for the input I appreciate it.

#5 Stephen

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Posted 12 April 2013 - 11:19 AM

Run some tests on small pint batches so you don't ruin the big batch. Approximate how much dry material is in your 5 gallon bucket, probably around 9,000 t0 10,000 grams depending on the recipe, and calculate that down to a pint. Better to estimate low, so you don't add too much of anything. Start with 3%, 6% and 10% additions of silica to fix the crazing. Once you've figured out how many grams of silica you need for the pint, convert it to the 5 gallon bucket amount. Remember to screen the big batch after adding the silica.


Roger that, if I do have to go as high as 10 do you think I will lower the melt much off of the 6 mark?

#6 Marcia Selsor

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Posted 12 April 2013 - 11:54 AM

I have cured crazing with as little as 1-3% silica and eliminated running by adding 2-5% epk.
You have to test and then guesstimate how much it is the bucket. Start with plastic cups of the mixed glaze. An 1 1/2" of mixed glaze will give you about 50 dry grams and the water. Take 5 cups at 1 1/2" and add your test %. 3 % of silica would be 1.5 grams to add to the cup. When you go to adjust the big bucket, start low like what you think could be about 50-75% of what you want. And work it up from there.
Test kilns are essential when you need them in a case like this.

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#7 Min

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Posted 12 April 2013 - 01:45 PM

Brongniart's formala works to estimate the amount of dry materials in a bucket of glaze. If you work the formula through then run your tests on a small amount of glaze (like a liter) then you will know the amount of silica to add to the full bucket. This will enable you get more acurate figures, once you have the adjusted silica figure for one bucket then you could adjust your other glazes to the same silica levels, with glaze software, that works to prevent your crazing. (assuming just adding silica will cure the crazing). I would run a series of tests to determine the coefficient of thermal expansion of your new clay so you can ballpark the figure you need to get to with your old glazes.

A link to Brongniart's formula is here: http://members.optus...a_made_easy.pdf

#8 neilestrick

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Posted 12 April 2013 - 03:12 PM


Run some tests on small pint batches so you don't ruin the big batch. Approximate how much dry material is in your 5 gallon bucket, probably around 9,000 t0 10,000 grams depending on the recipe, and calculate that down to a pint. Better to estimate low, so you don't add too much of anything. Start with 3%, 6% and 10% additions of silica to fix the crazing. Once you've figured out how many grams of silica you need for the pint, convert it to the 5 gallon bucket amount. Remember to screen the big batch after adding the silica.


Roger that, if I do have to go as high as 10 do you think I will lower the melt much off of the 6 mark?


Probably not. If it does it will be minor and could be easily adjusted with a very small increase in one of the fluxes.
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www.neilestrickgallery.com

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#9 Stephen

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Posted 15 April 2013 - 10:58 AM

Thanks everyone for your input on this. Over the weekend I was able to add 2.5% Silica and it cleared up the crazing completely on a gloss and on a matt it cleared it up except on 2 spots where the test tile had 2 very distinct cracks. I was tempted to up it another point but decided to test it on a couple of pieces this week and then do a stress test to see if they will hold up.


Thanks again to each of you for taking the time to help me with this!

#10 Biglou13

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Posted 15 April 2013 - 11:55 AM

Thanks everyone for your input on this. Over the weekend I was able to add 2.5% Silica and it cleared up the crazing completely on a gloss and on a matt it cleared it up except on 2 spots where the test tile had 2 very distinct cracks. I was tempted to up it another point but decided to test it on a couple of pieces this week and then do a stress test to see if they will hold up.


Thanks again to each of you for taking the time to help me with this!


What is stress test? How do you perform?
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#11 Stephen

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Posted 15 April 2013 - 02:09 PM


Thanks everyone for your input on this. Over the weekend I was able to add 2.5% Silica and it cleared up the crazing completely on a gloss and on a matt it cleared it up except on 2 spots where the test tile had 2 very distinct cracks. I was tempted to up it another point but decided to test it on a couple of pieces this week and then do a stress test to see if they will hold up.


Thanks again to each of you for taking the time to help me with this!


What is stress test? How do you perform?


sorry, I phrased that badly I guess, I was referring to doing some thermal shock test to make sure that the fit was in fact a good one and that the pieces will not just craze when exposed to extreme heat. These glazes will be used on mugs and such so I want to make sure the fit is right before I use up the glaze.

Thanks again everyone for the help. I was inclined to just dump in 10%, which it looks like would have been too much to accomplish my goal of correcting the fit with the least amount of disruption to the glaze formula as possible.




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