Wow the complaints and panic are coming in daily!
Duncan, Mayco, Gare and Amaco all got the toxic (but loveable) heavy-metal LEAD out of their products. Why? Most people using the glazes didn't know about the lead. Let's put that behind us and see what we can do today to get back to work.
We tested every low-fire clear glaze we could find and after much testing I decided they are equally okay. The problem is that of application, the old glazes you could slap them on and they looked great! Now not so much..
1. Bisque low fire clays to Cone 04, either leave the kiln lid ajar until 800 degrees F or use a vent system. . This is very important as you must burn the carbon out of the clay during the bisque firing so it will not affect the glaze on the glaze firing. Carbon trapped inside the clay comes out as ugly blisters in the glaze on the second firing. Take at least 6 hours for the kiln to fire, faster firing is uneven firing and it takes time for the carbon to burn off.
2. Thin your glaze, mix well and keep mixing as you use it. Use a good brush!!! Apply two even, thin coats. Non-toxic glaze is much less forgiving to application errors.
3. Glaze fire to Cone 06.
Problems like crazing (crackled glaze) and shivering (glaze falls off pot on cooling) are easily avoided. Follow the RULES! We cannot re-create shivering on our clay bodies unless we bisque to Cone 4 (not 04). Crazing is caused by way underfiring, say to Cone 08 bisque and glaze.
Red Clay is a problem, the lead glazes of the past wetted the clay (as if you got it wet) and this gave us the bright colors we loved. Now we have a tradeoff between a durable glaze (IN1001) and a less durable or softer glaze that gives a little brighter colors. (Mayco's S2100)
Another new problem is that the Talc mine in New York was shut down. All the glaze companies made their traditional underglazes out of this lovely white talc. The only mine left is in Texas and the talc is a dark grey color. All the big companies had to reformulate the clay bodies and underglazes at the same time as we were getting the (forgiving) lead out. Be careful about putting opaque underglazes on bisque without testing. New Mexico Clay has used the Texas Talc for 25 years and so don't have the re-formulating problems.