Saturday, April 6, 2013

The First Thing to Try

With 60-something minutes left, I am getting a jump-start on my project; reformulating my glazes to melt at ^6. I've been doing it in my head for a while now, of course, and today I am mixing up the first test that is not someone else's glaze (not that there's anything wrong with that!) but an attempt to alter my own glazes.

When something seems daunting, it's often best to try the easiest thing first. In this case the easiest thing is a one-for-one substitution, Nepheline Syenite for Custer Feldspar. A higher temp feldspar exchanged for a lower. This will work better for some glazes than others - I have one glaze that's  already 71% Neph Sy, so I'll have to try a different approach with that one - but what better place to start than with my current darling, Jim Brown's Blue, which goes like this:

Whiting        2800
Custer Spar  6000
EPK             1200

Cobalt             75
Rutile            400
At ^6, it might look like this:

Whiting        2800
Neph Sy  6000
EPK             1200

Cobalt             75
Rutile            400
Will it work? Well, I don't know! But it's starting point so my plan today is to  mix up versions of many of my glazes with that simple switch.

First test firing is April 24th.

11 comments:

DirtKicker Pottery said...

It looks like you accidently listed the same recipe for both.

I'm really looking forward to following your conversion work. Glaze chem is a challenge for me, but I really want to learn.

Lori Watts said...

Thanks! Fixed it.

June Perry said...

Subbing Neph Sy for a potspar will take the recipe down 2 cones. So if you glaze would have worked at cone 8, then this simple substitution may work at cone 6. If you wan to be more exact, you can download one of the free on line glaze bases and follow the limits for cone 6 and get a good chance of getting a good, safe, cone 6 version.

June Perry said...

Subbing Neph Sy for a potspar will take the recipe down 2 cones. So if you glaze would have worked at cone 8, then this simple substitution may work at cone 6. If you wan to be more exact, you can download one of the free on line glaze bases and follow the limits for cone 6 and get a good chance of getting a good, safe, cone 6 version.

Lori Watts said...

Hi June - The Neph-Sy suggestion came out of Val Cushing's notebook, so I think it's worth a try. It certainly won't be the only thing I am trying!

June Perry said...

Lori, I ran your original recipe with the Custer through my Insight, glaze chemistry program and the silica is even too low for a cone 6 recipe. So I used the limit formula for cone 6 and came up with this one. The calcium from the whiting is a bit high for cone 6 and like the original it has high expansion. I suspect this glaze may craze for you. I'm also posting a second one which I used with the cone 6 limits right on. So you might want to try these along with you Neph Sy substitution. The Neph sy sub makes it super low on silica and too high in alumina and the alumina silica ratio is going to be very, very low on the silica range.

#1 Cone 6 revision (just add your oxides as given in the original)

Custer feldspar 5565.86
whiting 2597.40
epk 742.11
Silica 1094.62

#2 Cone 6 within all cone 6 limits.

Untitled
Custer feldspar 7005.66
whiting 1585.94
epk 934.09
Silica 474.30



DirtKicker Pottery said...

Lori, Will your glaze testing be fired in an electric kiln?

Lori Watts said...

June - Thank you so much! I will definitely try these. It is sounding more and more like I need to get my hands on that insight software, or maybe digitalfire. My first purchase with the KS cash.

Cindy - Nope, these will be strictly reduction soda glazes. The softbrick in the electric kiln can't the corrosive soda - nevermind the elements!

June Perry said...

Lori, let me know if you have any other glazes you want to re-do for cone 6. I'll be happy to run them through insight for you and give you some insight into the safety, etc.

You have to expect that things will not be the same in soda. If a glaze crazed for you in oxidation or reduction, it will most probably craze a bit more from the soda. That's just one consideration. You original glaze and even my adjustment for cone 6 still have a pretty hight rate of expansion.

Know that high boron, high calcium and magnesium and high barium glazes will repel soda. Any glaze with about 15-20% or so of those ingredients in combination will be able to take the soda without fluxing the glaze so much to cause running off the pot. The colors will change from what you're used to because the extra sodium from bicarbonate of soda or soda ash it going to push those glazes more into an alkaline chemistry, therefore increasing the rate of expansion of the glaze, therefore increasing crazing.
There are some cone 6 soda glaze recipes on Julia Galloway's site; and you might want to try some of those.
One or two liner glazes (that's the easy part) and a couple of exterior glazes and one good flashing ship should be plenty for you to get started.
I don't know if you're planning on reducing, but high clay shinos and temmokus make good liners. Even temmoku with about 1% cobalt oxide added to the iron content will make it work in oxidation; and a high clay shino glaze will give you a cream color in light reduction/oxidation.
Oribes and rutile blues also work well as exterior glazes. Shinos in soda and reduction when you on the exterior just turn brownish, so they may not hold any interest for you.
Some glazes in books that come out glossy, are actually matt glazes that are way too low in Silica for safety for the cone they're listed for, and wind up working at a higher temperature because they're melting as a gloss because they're actually over fired matts. The glaze you posted is one such glaze. The calcium was a bit too high for cone 6 and the silica way too low for cone 6 and even more so for cone 10; and the ratio of alumina and silica was something in the 5 range which is what you want for a pretty matt glaze. The expansion was also quite high - in the 8 range; and for a good, safe, non crazing glaze, you're aiming for an expansion of 7 or even less in the case of porcelain bodies. One of the dangers of a glaze too low in silica is that lacking in silica, it can wind up being a very unsafe glaze if, for instance it contains copper. Such a glaze will release the copper into the food being cooked or served or stores in such a pot, because there's just not enough silica to hold the poisonous copper carb in solution, and any acid foods will release the copper. Any just washing those dishes over time, will cause the green to bleach out. Copper needs a lot of silica in a glaze to hold it in solution and to be safe no more than 4% should be used and maybe even less, if the glaze is used on the interior of a pot where food can be stored. Even if you don't add a potentially toxic colorant in the glaze, a glaze too low in silica will cause crazing and present other aesthetic and health problems. A glaze too low in silica is going to be a soft glaze, easily marked with cutlery.
Once you know the chemistry, you can use the glaze chemistry programs to create whatever you want; and you will never be at the effect of some mine closing, or material changing. You can just adjust it at the molecular level using glaze software or just learning to do it manually (but I definitely recommend the software are the time saving thing it is!).

Marian Williams said...

Thanks for the Jim brown Blue recipe! I've tried it but didn't have great success- trying it again! I fire cone 10 redux in a fibre gas kiln- so any suggestions? Apply thick? Is it runny? Thanks!

Lori Watts said...

Hi Marian - Yes, it runs like a mad bastard. If you apply it thickly, do so only on the upper half of the pot.

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