Identifying Raw Material in a Glaze

[Neil Fallon]

I am a Ceramics Arts Network subscriber, but for some reason I cannot post as a member, so I am posting as a guest for now. 

I mixed a clear glaze for a community clay space from a recipe I found on Glazy.org.  It is called Transparent 24 and includes only three ingredients.  Ferro Frit 3134 (45%) EPK (35%) and Silica (25%).  After testing the Specific Gravity, I landed on 130 to use this glaze for dipping.  The problem I have is that someone unfamiliar with self-mixed glazes (non-commercially available),  re-sieved the 3-gallons of glaze and threw away the material caught by the sieve that had settled out of the mixture.  I tested the glaze on a few pieces. It fires to a dull, clear glaze that looks and feels underfired.  I am far from a glaze chemist.  Besides throwing out the 3-gallons of glaze and remixing a new batch of glaze, is there any way to determine what raw material is left in the glaze?

Neil Fallon

[Min]

A few thoughts after looking at the recipe 

- having only frit, epk and silica there shouldn't have been anything that didn't go through a sieve, even a 100 mesh one.

- are you confident the kiln is reaching cone 6 (Glazy has this as a cone 6 glaze)

- boron is super high for a cone 6 glaze which means the glaze probably won't be very durable 

- how does the glaze behave in the bucket when you stir it? does it settle really fast and form a hard layer at the bottom of the bucket or is it well suspended and creamy?

- does the raw glaze crawl or crack if you apply it thickly to bisque (test piece will do for this test) 35% epk is a heck of a lot, I would expect the raw glaze to crack or crawl

-are you confident in your weighing of materials and the materials you used are actually what they should be?

BTW you are logged in if you are able to post, did you have another account or name you logged in here previously with?

[Neil Fallon]

Min,  thank you for your quick response.  Here are my answers to your questions (in bold).

having only frit, epk and silica there shouldn't have been anything that didn't go through a sieve, even a 100 mesh one. 

I understand the person that sieved the glaze didn’t use a powerful glaze mixer.  Just used a small cocktail mixer.  I can imagine the material that settled out on the bottom was never mixed into the liquid.  She described the material left in the bucket as “beach sand”.

- are you confident the kiln is reaching cone 6 (Glazy has this as a cone 6 glaze)

Yes, this is being fired in a L&L Kiln with cone packs to verify the temperature is reached by the kiln.

- boron is super high for a cone 6 glaze which means the glaze probably won't be very durable 

Good to know.  Thanks

- how does the glaze behave in the bucket when you stir it? does it settle really fast and form a hard layer at the bottom of the bucket or is it well suspended and creamy?

If mixed properly it does stay suspended.  Even after the bucket sits for a few days, the glaze material at the bottom of the bucket is not like cement.   I mixed the glaze so people could dip pieces.  If I dip a vessel into the bucket and pull it out after a count of one or two, it covers well and fires to a clear glaze on Cone 6 B-Mix clay.  I’m mostly covering underglaze designs

- does the raw glaze crawl or crack if you apply it thickly to bisque (test piece will do for this test) 35% epk is a heck of a lot, I would expect the raw glaze to crack or crawl

Yes, if the piece is dipped and left in the glaze too long, the glaze cracks as it dries on the piece. It also crawls on pieces where the glaze was applied too thick.

-are you confident in your weighing of materials and the materials you used are actually what they should be?

Yes.  Using a Ohaus Beam Scale.

Answering your questions has made me realize that I need to mix a different clear glaze that I know better. I chose this one from Glazy because it only had three ingredients.  The people at this community center are mostly beginners with clay and I thought this might be (1) more cost effective and (2) easier to mix.  Wrong!  

I’m assuming that the 3 gallons of “mystery glaze” is not usable in its current form.  If I get some time, I could experiment with the glaze to use in my own studio.

BTW you are logged in if you are able to post, did you have another account or name you logged in here previously with?

Thanks.  I realized that after I created as a guest, the system directed me to set up an account to be able to post.

[Min]

1 hour ago, Neil Fallon said:

She described the material left in the bucket as “beach sand”.

What mesh size is the sieve and the silica?

1 hour ago, Neil Fallon said:

Yes, if the piece is dipped and left in the glaze too long, the glaze cracks as it dries on the piece. It also crawls on pieces where the glaze was applied too thick.

Going forward if you have a glaze recipe with a large amount of kaolin like this recipe does then calcine part of it. Just a ballpark amount would be to have up to 20% kaolin and anything over that use calcined kaolin. (just put the kaolin in a shallow bowl and add it to your bisque fire) Calcining removes the chemically bound water from the kaolin so it will weigh less and reduce the shrinkage of the glaze thereby reducing cracking and crawling than uncalcined kaolin. Since the calcined kaolin has less water in it's weight you need to allow for this in the recipe. Explanation on how to rebalance the recipe  here if you need it.