What Is Wrong With This Recipe?

[rayaldridge]

I don't have any colemanite either.  I did buy a 50 lb sack of gerstley borate back when it started to disappear, and that's probably a lifetime supply for me, firing at Cone 8 and up.

 

Here's a quote I ran across on ClayArt:

 

"You can sub Colemanite for Gerstley, but not knowing the source of
your Colemanite, I would like to throw in my two cents worth.
Our Colemanite (South Africa) comes from Turkey and is
contaminated with Gypsum, I have watched that damn stuff get up
and walk off my pots! Personally, I would rather try and find a frit sub,
that is if you really hate the Gerstley. I love it, can't get it here and am
deeply grateful to the Clayarters who trade all sorts of odd things with
me for Gerstley.
Toni Martens Durban South Africa"

 

And this:

 

"I read your question about colmenite/gerstley borate with a sinking heart.
I was given some gerstley borate many years ago by an american friend and
had some super results with it. At the time you couldn't buy it in the UK
so I tried substituting colmanite - result? The glaze spat off the pot and
lay in a pool on the kiln shelf. Hope you have more success - I would love
to hear if substituting works for you - Carenza."

 

 

I would say this might be Paola's problem, except that she says the same happens when substituting frits for the colemanite.

[David Woodin]

There is a possibility that the bisque firing is not correct.  With the speed of the last ramp plus 30 minute soak you are firing somewhere between C08 and C07.  Also the program is too fast thru the carbon burn out area, 1000 deg F to 1100 deg F.  The glaze program is also too fast for the last ramp.  You should try to mimic the fixed programs in the manual for the kiln controller.

David

[Tyler Miller]

 

I think you're overfiring it.  You've got got a lot of flux there, not just boron.  I've got a mid-fire high boron glaze that will behave similarly.  Someone once explained to me that if you've got a lot of boron in higher temp glazes, it will want to separate itself out, causing blisters and bubbles.  It's clear that your glaze was doing some heavy duty bubbling and spitting in the firing.

 

If it were me, I'd up your clay content, lower your silica content, and see if you can get away with less boron.  

 

So it is all to do with phase separation? http://digitalfire.com/4sight/glossary/glossary_phase_separation.html

 

It's possible.   Boron in large quantities at temps above cone 3 can do strange things.

[David Woodin]

I substituted Gerstley Borate for the colmanite in your glaze and made a 8 tile test and added the GB back  into the glaze in 4 gram amounts, by tile 5=16 gram GB the surface was clear and glossy, tile 6=20 gram no blemish surface, tile 7=24 slight milkyness where thick no blemish, tile 8 some healed over pin holes, 8=28 grams.  Self supporting cone 6@ 90 degree bend, clay laguna Amador stoneware. No faults at all. I don't know what this means in your case, clay? bisque not burning out the carbon material in clay? glaze firing too fast? Ware not dry enough after glazing to be fired?

David

[Min]

I substituted Gerstley Borate for the colmanite in your glaze and made a 8 tile test and added the GB back  into the glaze in 4 gram amounts, by tile 5=16 gram GB the surface was clear and glossy, tile 6=20 gram no blemish surface, tile 7=24 slight milkyness where thick no blemish, tile 8 some healed over pin holes, 8=28 grams.  Self supporting cone 6@ 90 degree bend, clay laguna Amador stoneware. No faults at all. I don't know what this means in your case, clay? bisque not burning out the carbon material in clay? glaze firing too fast? Ware not dry enough after glazing to be fired?

David

 

Good on you for doing this David!

 

So you got the boron levels up with GB as high as they would be with colmanite? I know when I first started with clay colemanite was no longer available and potters were just subbing GB for it but when I look up the composition of them now the GB has much lower boron than the colemanite. Not sure which figures would be the ones to use? plus how accurate our figures would be compared to the materials available to Paola? I'm thinking there is more than one thing going on here like you suggested. 

[rayaldridge]

If Paola is in Europe, she might have some of the bad colemanite referenced in those ClayArt posts.

 

But it still doesn't explain why it happens when she substitutes frit for the colemanite, unless I've misunderstood her.

[Cadaola]

I am in europe but not at home in these days... i realize looking back at the recipe that i forgot to mention the coloring oxides i actually used for glazing the pot and that may affect the final result... In the base recipe i added 0.4% chrome oxide 5% tin and 6% rutile.

Thank you David for your tests! I will read your post carefully when i an back home trying to understand... It is possibile that when i substituted colemanite with Frit and i had the spitting problem, it was caused by thick application... Because i am discovering now that i tend generally to do so and only recently i am correcting this bad attitude.

Rayaldridge, you did not misunderstand me.

[Cadaola]

http://www.sciencedirect.com/science/article/pii/089268759500030T

 

I found this article about calcinating colemanite...

[Cadaola]

http://www.sciencedirect.com/science/article/pii/S0032591004001196

 

...and this one.

[Cadaola]

I substituted Gerstley Borate for the colmanite in your glaze and made a 8 tile test and added the GB back  into the glaze in 4 gram amounts, by tile 5=16 gram GB the surface was clear and glossy, tile 6=20 gram no blemish surface, tile 7=24 slight milkyness where thick no blemish, tile 8 some healed over pin holes, 8=28 grams.  Self supporting cone 6@ 90 degree bend, clay laguna Amador stoneware. No faults at all. I don't know what this means in your case, clay? bisque not burning out the carbon material in clay? glaze firing too fast? Ware not dry enough after glazing to be fired?

David

In my test tiles the glaze behave nicely. I suspect clay, carbon material and glaze firing too fast being guilty, so I will try to bisque and glaze fire differently. I think that if the problem is carbon material not burned away I should have the same problem also with other glazes that istead work properly, even if it is true that sometimes I have a small crawl problems with a transparent glaze... I will let you know.

 

If you like the result and want to use this glaze, in my test tile I obtained a blue I really like by adding 2% manganese, 0.4% chrome ox. and 5% rutile. (I post a picture) I did not try it yet on pots, so I don't know how it will behave... All the best! Paola