Low fire glazes at cone 5/6

[Hailey]

So I’ve been finding some low fire (05/06) glazes for really cheap lately. They all say that they can be fired up to cone 5/6 but that the color will be slightly different. But they only guarantee food safety at the low fire temperature. By firing to a higher temperature am I compromising food safety? Is that something I should be worried about? Or is it just something they say for legal matters?

[Babs]

If you go this way, fire on small shelf savers. Going  higher may make some glazes run.

Re. Safe with food, may be safer higher in some cases.

It may be the man. only tested at tthe temp. that the glaze is manufactured for.

[LeeU]

Be prepared for a variety of color changes when firing 05/06 to 5/6. Many will wash out or become muddy-ish. Some will stay true, but ya never know. Check the product detailed info on the manufacturer's site AND on the suppliers' sites, because they do not always match. The better labels and descriptions will tell you  something like "this  05/06 deep purple becomes pale pink at 5/6".

[Bill Kielb]

18 hours ago, Hailey said:

But they only guarantee food safety at the low fire temperature

I would take them at their word Re: not being guaranteed food safe fired higher.

[Min]

On 6/4/2021 at 12:45 PM, Hailey said:

But they only guarantee food safety at the low fire temperature.

Think defining the term "food safety" is the at the crux of this. I don't know which commercial glazes you are using, Mayco's  definition of "food safe":

"“Food Safe” refers to the glaze in its final, fired state. “Food Safe” is a regulation and is defined by the Compliance Policy Guide, which is maintained by the FDA and only applies to lead and cadmium release. “Food Safe” refers to the glaze in its final, fired state. If a product has a food safe designation, we have conducted testing on the individual glaze (or similar in formulation), which has passed standards for food safety when used according manufacturer’s direction. Finishedware producers bear responsibility for food safe testing their ware being sold into commerce. "  

Regulations from FDA laws or regulations, primarily “Food Safe”. Specific to ceramic products and supplies: 

"CPG Sec. 545.400 Pottery (Ceramics); Import and Domestic – Cadmium Contamination: delineates the acceptable test methods and levels of cadmium leach that is permissible 

CPG Sec. 545.450 Pottery (Ceramics); Import and Domestic – Lead Contamination: delineates the acceptable test methods and levels of lead leach that is permissible 

Please note: it is NOT illegal for a foodware item to contain traces of lead or cadmium as long as these substances test below the thresholds established in these guidelines."

So, in theory a very high manganese, copper, cobalt etc glaze would comply with the FDA regulations, however it isn't going to be a good choice for a durable glaze for functional work. Some glaze manufactures muddy the waters even more by having a "Dinnerware Safe" label, Mayco's definition of Dinnerware safe:

"“Dinnerware Safe” indicates the fired glaze surface meets the FDA standards for food safe, the fired surface is free of surface texture that could potentially trap bacterial, and the fired surface is chemically durable."

You need to use some common sense when choosing glazes. The glaze below is Amaco's Saturation Metallic and they list it as "food safe". It's obviously heavily loaded with colouring oxides and isn't one I would even consider using for functional ware. If in doubt use a clear or white liner glaze and fire to cone recommended.