Pottery/Cone 5-6 Glaze on Cone 10 Clay Body
I have Highwater Phoenix and Half & Half clay. I bought Speedball high fire glaze. Once I got home I noticed that the glaze says cone 5-6 The clay firing is listed as cone 7-10 (although I have always fired to cone 10). Do you know if that glaze will perform at cone 10? I have found conflicted comments online. I do have a question in to Speedball as well.
I read that absorption for Phoenix at cone 6 is 3.9%. I'm trying to figure out if firing higher than cone 6 will ruin the glaze. I don't want to fire that low. I want the pieces to be food safe.
Thank you for your recent questions. Unfortunately, the glaze you have selected will not fire to Cone 10. The amount of flux ingredients in the glaze prevents the glaze from being fired at high fire without burning off the flux ingredients. The alumina and silica ingredients that make up the refractory and glass forming part of the glaze then will turn to a powdery substance and become a chalking surface on the clay. Furthermore, there will be no fusion between the clay and the glaze body, which insures the glaze is part of the clay body when the firing is complete and renders the piece safe to use as a functional product.
You discussed firing at high fire to render a piece food safe. However, it is not the temperature or Cone you reach that renders a piece food safe, it is the type of glaze you use as a surface coverage and the clay it is adhered to as well as whether or not the glaze is fired to maturity that will render a piece safe to use with food products.
Commercial glazes are purchased as toxic or non-toxic surfaces. Toxic surfaces are glazes that contain raw glaze materials such as lead and barium that are often used to create extremely bright colors at lower firing and therefore prevents the toxic ingredients from being burned off during the firing, even when reaching maturing temperature. Lead, for instance, when fired above it's recommended melting temperature will burn off and therefore, does not do it's intended job, make a bright color or allow the glaze to be fired at a low temperature. This is why lead is not used in functional glazes as it is too risky during the firing process to achieve the desired effects and still have a glaze that is food safe.
Additional toxic glazes can be higher fire glazes but are primarily glazes that have surface effects that render the piece unsafe because of the texture of the glaze. This includes crystalline and crackle glazes. These glazes allow bacteria to enter the surface cracks during functional use and can cause sickness for the person using them.
Functional or non-toxic glazes come in many firing ranges, however, they are selected based on the type of clay you are using and the Cone maturing temperature required to create the desired glaze surface. Because of this, when you purchase a clay body, you want to purchase a glaze or glazes that fire to the Cone maturing temperature of the clay body. In your example, you will want to purchase a glaze that matures at cone 10.
The conflict you are probably seeing online is where a potter uses a lower fired glaze on a high fire clay body to achieve the effects he/she wants in color, surface texture, etc. But what they are speaking of does not pertain to functional ware. Ware that is going to be used with food products must be fired correctly to prevent illness from bacteria and to assure raw glaze materials within the glaze is melded as intended.
Another issue is the fusion of the glaze to the clay body. Where potters use a lower fired glaze on a high fire clay body, they are not achieving the fusion between the surface of the clay and glaze as it should be. This is not a completely wrong process, but it does create a piece that is weaker in structure and is definitely not food safe. Because the glaze is not fused to the clay body, it can chip, even minutely, causing bacteria to enter the clay body.
As a potter, it is our responsibility to create functional ware that is safe for the public to use. Years ago, the use of lead in glazes was not understood as it is today and many people used pottery ware that was lead based, making many people sick. Just as it did for those who used household paint on baby cribs and window panes. Because we know today that lead is unsafe for functional ware, we do not use it in any glaze that we are going to put on functional ware.
So it is also true as to the methods and types of clay and glazes we use to produce functional ware. The clay and glaze should "fit" each other and mature at the correct Cone maturing temperature. The firing methods we use should reflect the correct method needed to reach the desired Cone maturing temperature to render the pottery safe to use.
My suggestion to you is to purchase a glaze that is for Cone 10 clay so that you can fire to cone 10. However, remember that you do not have to purchase a Cone 10 clay to render functional ware food safe. Purchasing Cone 5 or Cone 6 clay and firing to the maturing temperature is just as safe as the clay has been formulated to mature at that Cone temperature. Therefore, your glaze that you have that matures at Cone 5-6 can be used with the Cone 5 or Cone 6 clay and you can have beautiful functional ware at a much lower firing, saving you money on firing costs and opening your pottery ware up to much more beautiful glaze colors that you can not achieve at high fire temperatures.
I hope this helps you. If you have further questions, feel free to contact me any time. I am always at your service. Much luck to you.
Ms. Ti Phillips
Earth Stoke 'N Fire Pottery Studio and Artist Retreat