Comment

Comment by Norm Stuart on May 10, 2013 at 3:07pm

Glazes which are mostly frit obviously need something added to suspend the glaze and harden the glaze when it dries. The first choice for most would be 2% white-firing bentonite, as it's inorganic and won't mold.

I learned from experience that the chemistry of bentonite can destroy the fired result of some frit glazes. Two examples are the leaded low-fire Ferro Frits CM-940 cadmium yellow and CM-941 cadmium red. Both frits are a complete glaze. Adding even small amounts of bentonite or kaolin results in a black porous glaze resembling lava which leeches lead. As a paint pigment cadium red can even be turned black by exposure to sunlight.

The remaining alternative is to suspend and harden the glaze is an organic ingredient like CMC Gum or Xanthan Gum because almost all of the gum components vaporize in the kiln leaving nothing to affect the glaze chemistry. I prefer Xanthan because it's easier to dissolve in water and makes a thicker slurry with less gum. This way I can pre-mix the the dry glaze ready to add to water.

The "Olive Speckle ^06" glaze is far less sensitive than Ferro Frit CM-941, but adding 2% bentonite will produce a different looking glaze than the one made with only Xanthan gum added to the 90% 5301 frit and 10% nickel carbonate.

The version I made for ^6 adds 20% silica, 2% bentonite, and increases the nickel oxide by 22%. In this glaze, fired to a higher temperature, the bentonite and the xanthan gum versions are indistinguishable.

You're right, there's definitely a different audience here than at our studio where I'm asked repeatedly to explain what each ingredient consists of and why we use it. I apologize for adding the explanation of what xanthan gum is.

Ferro Frit CM-941  fired at ^06

Comment by Shine Chisholm on May 10, 2013 at 6:38am

Because xanthan gum is a biological agent, I wouldn't expect to find it in a glaze. I've seen it used as a brushing agent, but only in the quantity of glaze meant to be used immediately.

Norm, I do understand the chemical purpose of xanthan gum in food. Gluten is the protein that makes certain grains elastic and sticky. Being gluten-free and limited in the financial resources department, I've had to make most of my food from scratch for the past several years. My professional experience with chemistry has helped a great deal.

I didn't go into it because I didn't feel like this thread was the place for it. Your response felt condescending, but I'm going to assume that you didn't mean it to be.

Comment by Norm Stuart on May 9, 2013 at 10:53am

Xanthan gum dissolves far easier than CMC Gum does and I find it provides superior glaze properties. Both require bleach or other preservative to prevent mold growth over time.

Xanthan gum is a biological product produced from a carbohydrate slurry by Xanthomonas campestris bacteria. After a few days of growth the slurry is filtered and the xanthan gum is precipitated out by the addition of isopropanol alcohol.

I sometimes use Xanthan Gum in salad dressings where it gels the vinegar, giving it the same viscosity as olive oil.  Virtually all commercial salad dressings or dry mixes list Xanthan gum as an ingredient for the same reason.

Comment by Shine Chisholm on May 8, 2013 at 7:37pm

xanthan gum? I use that in gluten-free flour!

Comment by Norm Stuart on April 13, 2013 at 9:50pm

This is the same ^06 glaze shown in this topic header: Ferro Frit 5301 with 10% Nickel Carbonate,

But with 20% Silica added - then fired to Cone 6 using our 50F/hour slow-cool between 1,800F and 1,500F.

One white clay on the left, and brown clay on the right.

Comment by Norm Stuart on April 13, 2013 at 1:09am

According to the 2004 "Potter's Dictionary" the crystals are haematite, made of pure Fe2O3 red iron oxide. I notice some use up to 30% iron oxide.

http://books.google.com/books?id=TApnGTVLwxAC&pg=PA17&dq=wh...

Comment by Norm Stuart on April 13, 2013 at 1:07am

You'll notice none of these samples,  either bisque-fired or Cone 6 fires have the beautiful golden brown you achieve, even on white clay. The sparkles are there, but when not in the bright light the glazes are black, with the exception of the Spanish Iron Oxide fired to ^04.

The 3269 frit 100% with 16% crylolite and 20% Spanish Iron Oxide, fired to ^06, is so far the closest in color to yours.

The results are interesting given the different levels of iron in each type of iron oxide.

72.4% __ Iron Oxide - Black - Fe3O4
69.9% -- Red Iron Oxide Red - Fe2O3
61.5% __ Yellow Iron Oxide  - I haven't tried this yet.
60.8% __ Iron Oxide - Spanish

Tomorrow I'll unload a Spanish Iron 24% with 100% 3501 and 24.8% Spanish Iron, fired to ^6, but this time our usual firing schedule of 30 minute hold at the top followed by a 50F slow cool between 1,800F and 1,500F

 

I like the lighter color glaze melt the 60.8% iron content Spanish Oxide produces, adding less iron and extra silica.

Next best I like the high purity precipitate with 69.9% Iron.

Black Iron Oxide with the highest level of iron 72.4%, gives the least amount of  of sparkly crystals

 

Comment by Norm Stuart on April 12, 2013 at 9:36pm

Unattractive "test tiles" as I'm down to using abandoned bisqued pieces.

Ferro Frit 5301 100%, 20% Iron Oxide,

^04 first row on white clay,

^6 second row with 20% extra silica on brown clay.

Second photo 10% and 20% added silica on white clay, clearly not enough iron left in the melt.

It's amazing to see how far the fluorine 5301 frit has migrated into unglazed areas, especially with only 10% added silica.

Comment by Norm Stuart on April 11, 2013 at 8:32pm

I added 10% extra silica and 20% extra silica to my three low fire glazes. The 20% flowed just a little too much, the 10% extra silica flowed way too much, so I think adding 24% Silica would be perfect to make these low-fire glazes into ^6.

I now realize if I increase the glaze by 24% silica also I needed to increase the amount of oxide colorant by 24% as well. That seems pretty obvious in retrospect.

Using the same 10% nickel carbonate with 20% extra silica there was less green crystallization as there was less nickel to go around. The 3269 frit with 0.5% chrome oxide, which Laguna calls Tang Lime Crackle, has slightly fewer crackles with 20% silica, but is far better at staying where you put it unlike the test ring below which fused with the pancake.

The Aventurine glaze came out dark and sparkly on brown clay, looking nearly identical to the bisque temperature aventurine without the extra silica, and similar to the tile below with 3269 frit with 16% cryolite added.

But the combination of being 24% short on iron oxide the melt lost so much iron to the white clay that I ended up with an amber glass with no aventurine.

One constant theme seems to be I get better results with Spanish Iron Oxide which gives a lighter amber xolor yet fully aventurine, and good result with the red precipitate iron oxide which is darker. The Laguna red iron oxide gives a bad result as does the black iron oxide.

Comment by Norm Stuart on April 9, 2013 at 4:11pm

Off the top of my head I added 10% EPK, which doesn't seem at all sensible now.

Upon reflection it seems I should add only 20% to 25% Silica to make it more refractory, with a Silica/Alumina ratio similar to other glossy high fire glazes.

I'll also try adding 20% and 25% Silica to a "Tang Lime Crackle Glaze" which is a little too runny at ^6. With slow-cooling this glaze turns an ugly opaque mustard color. The only problem I see is the extra Silica will bring down the glaze expansion ratio, which may mitigate the crackle effect.

100% Ferro Frit 3269

0.5%  Chrome Carbonate

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