Durable (i mean food safe) low fire glazes

[Retxy]

Hi there smart folks!

I am hoping to see if anyone can recommend any books or resources (free is nice, but not required) that explain limit formulas for the development of durable/food safe low fire glazes. I now feel confident after having read mastering cone 6 glazes that I am at least in the ballpark of safety when formulating glazes for midfire temps ( with testing to follow of course). But I have zero clue as to how to go about formulating recipes for low fire, where the stakes seem higher given clay isn't vitreous. I really wish that I could keep working with stoneware, but the cost to fire is now prohibitive for my life circumstance, so trying to see what the world of low fire can do for me so I don't have to give up the only source of sanity now left to me. 

Thanks!

Ret

[Min]

Good article on Limits / Targets here. This article includes a lengthy bibliography if you need to go more in depth for further reading of the  researchers referenced including (but not limited to) Seger, Parmalee, Eppler & Rhodes and many others. 

Term "Limits" is a bit misleading as there are many glazes that fall outside the limits but are still good glazes, calcium or magnesium mattes for example.  "Target" is perhaps a better descriptive. In the article I linked on page 10 there are 8 points that are really the key to formulating a durable glaze. Really notable is having as much alumina plus silica as will be taken into the melt is really important as is not overloading the boron.

I would start off with finding which clays are available to you and how high you can fire them. Case in point is there is an earthenware that is available where I live that when taken to Cone 02 has an absorbency of 1.6%  If you can find an earthenware that can be fired to a higher cone than typical earthenware is without bloating / slumping / becoming brittle that would solve the high absorption that most earthenware bodies have when fired to typical cones. Unless it's a fritware body you will be looking at earthenware red bodies not white lowfire bodies.

Another thing I would look at is using terra sig on the bottoms of pots if you cannot get the absorption down low enough to prevent leaking pots. You might be able to use terra sig in place of glazing the entire pot and stilting. Design also will come into play. If you have a good foot ring with minimal unglazed surface area and glaze inside the foot ring that will minimize water absorption.

A shorter article on Limits here, chart from this page below. 

 

[Retxy]

Min ---you as usual are a genius. I hadn't even considered the implications of the unglazed bases of my pieces. I found a book written by James Chappell in which he gives some recipes for ware that is vitreous at cone 06... strange but I am super excited to try. I will check out the article above as the book does give a few glaze recipes that will work at this temperature but the targets above will help me branch out from there. THANK YOU AS ALWAYS for being willing to share your knowledge!!

[Min]

@Retxy,

At first I wasn't comfortable making a comment and I kinda feel quite awkward writing this but here goes...

After you made your comments to me I would now like to bounce them back to you. It's not about having X amount of knowledge or being able to connect dots or having a good memory that is important (well it helps) with this field that has so very many facets to understand. In the attempt to understand even a small fraction of it people asking questions leads to growth for all of us here, regardless of years of experience or having a good memory and then being able to apply things learned.

So my thanks go to you and everyone else for asking questions and wanting to find out about the "whys". It keeps us all thinking, to me this is what is important and matters.

Edited September 28, 2022 by Min

[Retxy]

Thank you so much for that welcoming attitude. I'm glad you went out of your comfort zone to say that. Makes me feel less bad asking all  of this stuff. I have a science background, but I really wish I had studied chem and not biology every time I delve into this subject. You're right, it's overwhelming!! Thank you for being so kind as to share your knowledge. It's so reassuring to have mentors and colleagues in clay that are willing to help each other out. Thanks for being one of those Min!!!!

D

On 9/26/2022 at 12:44 PM, Min said:

Good article on Limits / Targets here. This article includes a lengthy bibliography if you need to go more in depth for further reading of the  researchers referenced including (but not limited to) Seger, Parmalee, Eppler & Rhodes and many others. 

Term "Limits" is a bit misleading as there are many glazes that fall outside the limits but are still good glazes, calcium or magnesium mattes for example.  "Target" is perhaps a better descriptive. In the article I linked on page 10 there are 8 points that are really the key to formulating a durable glaze. Really notable is having as much alumina plus silica as will be taken into the melt is really important as is not overloading the boron.

I would start off with finding which clays are available to you and how high you can fire them. Case in point is there is an earthenware that is available where I live that when taken to Cone 02 has an absorbency of 1.6%  If you can find an earthenware that can be fired to a higher cone than typical earthenware is without bloating / slumping / becoming brittle that would solve the high absorption that most earthenware bodies have when fired to typical cones. Unless it's a fritware body you will be looking at earthenware red bodies not white lowfire bodies.

Another thing I would look at is using terra sig on the bottoms of pots if you cannot get the absorption down low enough to prevent leaking pots. You might be able to use terra sig in place of glazing the entire pot and stilting. Design also will come into play. If you have a good foot ring with minimal unglazed surface area and glaze inside the foot ring that will minimize water absorption.

A shorter article on Limits here, chart from this page below. 

 

 

[Kelly in AK]

Emboldened by the conversation.

I’m a little primitive because I haven’t done the work to communicate at the “mol” level yet. It’s on my list! I do understand, however, the importance of balancing the elements in a glaze, making it fit, and making it durable. So my contribution is only based on empirical experience and centers around making functional pots at low fire. It’s what I’m doing right now. 

Many earthenware clays develop significant strength and decreased porosity just above cone 04. Of course many earthenware bodies never get close to becoming vitreous, but some do. One trouble is that the range they mature at is very narrow. Half a cone too high and all kinds of trouble sets in, half a cone too low and it’s still very porous. Another issue is that when the clay is at its sweet spot temperature wise it’s also beginning to decompose and off gas, creating pinholes in the glaze. Frit fluxed clay bodies such as Tony Hansen’s Zero 3 body or Zero 4 porcelain are another alternative. It’s a narrow path, but it certainly exists. 

I’ve had success with my local earthenware at cone 03, producing pots that have less than than 1% absorption with glazes that fit. I mostly eliminated pinholing by using a drop and hold firing schedule. Some people fire their bisque hotter than their glaze firing to solve that problem. I fire with gas and admit I have to watch the kiln like a hawk near the end. 

I rely on three tests to determine how functional a glaze is:

Regular use. I use my pots just like I want others to use them. I expect to catch problems first in my own kitchen.

The dishwasher. After watching the life cycle of many glazes over time I know that low fire glazes can be formulated to be resistant to alkali attack. If it’s still shiny after a year of use I have confidence in that glaze. As an aside, I’ve seen a few cone 6 glazes become frosty and mat before the low fire ones I use. That’s just to reiterate a high firing range is no guarantee of durability. 

Acid, vinegar, wine. Ok, I’ll confess the wine test happens a lot more often than the vinegar test, but again, there are low fire glazes that stay shiny and true to color after soaking in acid. 

Two glazes I use regularly are:

White-

Frit 3134  40

Frit 3124  40

Grolleg    10

Local clay  10

Add:

Zircopax  10

 

Red (in reduction) or blue green-

3134  80

OM4  20

Add:

Zircopax  10

Copper carb  1

This one crazes a bit on my clay body, big crazes, not crackly. It’s held up well though as far as durability. 

I flocculate the glazes with Epsom salts to the right viscosity, which helps with application. My pots are a little less porous than regular bisque ware. All the pots get a few layers of terra sig on the outside and I mostly glaze only the interior and rims. 

[Retxy]

Kelly in AK, thank you so much for this information! Sorry I'm so late to the game, have been ill.  I appreciate the information and will try to apply stat! Thanks!!

On 9/28/2022 at 11:51 PM, Kelly in AK said:

Emboldened by the conversation.

I’m a little primitive because I haven’t done the work to communicate at the “mol” level yet. It’s on my list! I do understand, however, the importance of balancing the elements in a glaze, making it fit, and making it durable. So my contribution is only based on empirical experience and centers around making functional pots at low fire. It’s what I’m doing right now. 

Many earthenware clays develop significant strength and decreased porosity just above cone 04. Of course many earthenware bodies never get close to becoming vitreous, but some do. One trouble is that the range they mature at is very narrow. Half a cone too high and all kinds of trouble sets in, half a cone too low and it’s still very porous. Another issue is that when the clay is at its sweet spot temperature wise it’s also beginning to decompose and off gas, creating pinholes in the glaze. Frit fluxed clay bodies such as Tony Hansen’s Zero 3 body or Zero 4 porcelain are another alternative. It’s a narrow path, but it certainly exists. 

I’ve had success with my local earthenware at cone 03, producing pots that have less than than 1% absorption with glazes that fit. I mostly eliminated pinholing by using a drop and hold firing schedule. Some people fire their bisque hotter than their glaze firing to solve that problem. I fire with gas and admit I have to watch the kiln like a hawk near the end. 

I rely on three tests to determine how functional a glaze is:

Regular use. I use my pots just like I want others to use them. I expect to catch problems first in my own kitchen.

The dishwasher. After watching the life cycle of many glazes over time I know that low fire glazes can be formulated to be resistant to alkali attack. If it’s still shiny after a year of use I have confidence in that glaze. As an aside, I’ve seen a few cone 6 glazes become frosty and mat before the low fire ones I use. That’s just to reiterate a high firing range is no guarantee of durability. 

Acid, vinegar, wine. Ok, I’ll confess the wine test happens a lot more often than the vinegar test, but again, there are low fire glazes that stay shiny and true to color after soaking in acid. 

Two glazes I use regularly are:

White-

Frit 3134  40

Frit 3124  40

Grolleg    10

Local clay  10

Add:

Zircopax  10

 

Red (in reduction) or blue green-

3134  80

OM4  20

Add:

Zircopax  10

Copper carb  1

This one crazes a bit on my clay body, big crazes, not crackly. It’s held up well though as far as durability. 

I flocculate the glazes with Epsom salts to the right viscosity, which helps with application. My pots are a little less porous than regular bisque ware. All the pots get a few layers of terra sig on the outside and I mostly glaze only the interior and rims.