Strontium Crystal Magic Cool- Food Safe?

[Caroline E]

Hi all, 

I came across a glazy.com page by lakeside pottery that stated Strontium Crystal Magic Cool ^6 is not food safe. I haven’t seen this information anywhere else and it looks like a popular glaze widely used on functional ware. Does anybody have any information on this? What would make it not food safe?

Here’s the link:

https://glazy.org/recipes/4301#

 

thanks very much

 

Caroline 

[Min]

First off have a look at the materials and their amounts in the recipe itself. I re-totalled the recipe from the original 60.60 to 100 as that is the standard base amount. When we apply a standard amount it makes it easier to notice possible areas of concern. Top part of the screenshot below is the Glazy recipe re-totalled to 100.

First thing that jumps out is there is no silica added to the recipe. Yes, there will be silica in the formula from the feldspar etc but probably not enough. (more on this later) Second thing that jumps out is the amount of titanium in the recipe, nearly 12%, this is a huge amount! Third thing, is approx 40% feldspar for a ^6 glaze is getting quite high. Put these things together and you have a recipe that won't be durable (low silica), probably won't melt properly at cone 6 and is overloaded with titanium.

I know this glaze and the "warm" version of it are very popular at cone 6. It's important to note also that it isn't used as a standalone glaze, it is used as a layering glaze with one or more other glazes. What the combined chemistry is of the layered glaze is what needs to be looked at, will those be durable, I don't know, each combination would need to be looked at.

BTW, giving credit where credit is due, this glaze started off as a Tom and Elaine Coleman ^10 glaze called Yellow Crystal Matte. Barium was removed from original glaze and strontium subbed for it, the gerstley borate was subbed with the Ferro 3124 and the zinc removed.

 

 

[Callie Beller Diesel]

Steven Hill was the one I encountered who did the strontium substitution and other alterations years ago. I took a workshop from him sometime around 2003, and the SCM glazes were on his glaze handout sheet. He was careful to say that SCM was never meant to be a liner glaze, and was intended to be a base for other glazes to be sprayed over for special effects. It makes sense when you look at his work.

He substituted the strontium in to the Coleman recipe because barium was becoming an anathema at the time, but also acknowledged that both materials are on the soft side. Both can be prone to various forms of erosion when they’re out of balance. 

I was going to add this link just as an illustration of his work, but he also talks a bit about SCM on this page. 

[Caroline E]

@Callie Beller Diesel and @Min Thank you very much for your responses! I have some follow up questions. 

Would the glaze be food safe if it was only applied on the outside? From what I can gather from your response, it is not food safe because it does not properly melt at ^6? I also wanted to ask you about the inclusion of lithium carbonate. I realize lithium carbonate is a toxic material. Does any ingredient labeled "toxic" mean that it can not be included in a food safe glaze? When any material is "toxic", is the concern in the powdered form or the fired form?

Also, can you please expand on your mention of the glaze not including silica and what that does to the nature of the glaze? 

 

Thank you very much 

 

Caroline

[Callie Beller Diesel]

Toxicity is a whole entire subject by itself. Mostly we have to start with definitions to help clarify. 

Tl;dr, yes you can put this glaze on the outside of a pot and it will be fine. It’s not touching food. It melts fine at cone 6, it’s just a “soft” glaze.

When a raw material is labelled toxic, that means it’s toxic in its current form. Once you put a material into a glaze with other materials and fire it, it becomes another material entirely, with different properties. If you’re using a commericallly mixed glaze that is labelled non-toxic, that means it’s not toxic in the unfired state. (It’s probably not toxic in the fired state either, but you’d have to test it to confirm, and manufacturers can’t say that it is for sure under all circumstances without legal problems. People do weird and unexpected things.)

When we’re talking about glazes, most often the finished glaze is more safe than, say, raw lithium. So potters should be concerned about their own health and safety first, and the end user second. Materials like lithium can be handled safely with sensible studio hygiene practices, and can be beneficial in a lot of good glazes.

When knowledgeable glaze chemists talk about food safety, we aren’t talking about whether a glaze will leach Bad Things (TM) into food. This is a very infrequent scenario outside of lead use, and lead hasn’t been used in most handmade pottery for decades now. But we are thinking about craftsmanship and durability. We’re asking questions like Does this glaze have flaws like crazing or pinholing, that can affect structural integrity or possibly harbour bacteria? Does this glaze scratch, or does it cutlery mark? Does it stain easily? Is it easy to clean? If those problems are solved, leaching is usually a non-issue. 

Re the silica question: You need 3 types of components in the right proportions in a base glaze for durability: silica to form glass, alumina for strength and to make it not run too badly, and different metals that make silica and alumina melt at much lower temperatures than they do by themselves (fluxes). Without enough silica or alumina, or if those things are in the wrong proportions, the glaze may react to acids found in many drinks, bases like in dishwasher detergent, or may scratch or cutlery mark. SCM doesn’t supply enough silica for durability, but the strontium is also a problem.

Within the flux category, there’s a bunch of stuff that’s a good idea to use in combination, because each material has beneficial and undesirable properties, and mixing them can bring out the best (or sometimes worst) of both worlds. For example, sodium is a strong flux, which is needed to bring down the melting temperature of silica and alumina, and is in most glazes. However, it tends to make glazes craze when by itself. But if you mix it with calcium (or barium or strontium or lithium and a few others), you can still get the melting, but less crazing.

The calcium/magnesium/strontium etc group of fluxes also give you different colour responses, or can help create matte effects if the proportions are high enough. One of the drawbacks of using strontium or barium though is that neither is very high on the Moh hardness scale, so if you use too much of it, the resulting glaze can be prone to scratching, staining or other flaws. But it helps create that really cool matte surface because it promotes crystallization, so sometimes creative folks make trade offs. 

[Min]

I prefer to use the term "durable" rather than food safe when talking about glazes. There isn't one agreed upon definition of what makes a glaze "food safe". In North America if a glaze does not contain lead or cadmium it can be labelled food safe. This is a very wide definition and adding something along the lines of what Mayco says "Finishedware producers bear responsibility for food safe testing their ware being sold into commerce." IMHO isn't good enough. Like Callie said Strontium Crystal Magic (SCM) is  fine on the outside of pots where your mouth or food won't be in contact with it.

Re lithium and toxicity, just adding a little bit. Topic of lithium and toxicity has come up in many discussions in many forums etc over the years. My impression of the reading I've done is it might not be an issue for most people but for those on lithium therapy it can be, both in the process of mixing glazes and leaching glazes.  Poison is in the dosage scenario.

With all due respect to Callie I found SCM quite rough, dry and unmelted. You could try it by itself on a test tile and see how you find it but it's a bit of a moot point because it isn't used by itself.  The overload of titanium is what makes the crystals/variegation when layered with other glazes. A good way to get interesting glaze reactions is to layer 2 or more dissimilar glazes, what one glaze lacks or has a surplus of reacts with the other glaze, this often results in interesting visual texture. SCM is loaded with fluxes to react with other glazes, that's part of the reason why it works.

Re silica and glaze durability, having as much silica and alumina in a glaze is one of the ways to ensure a durable glaze.  For cone 6 having at least 2.5 molar of silica is a good starting place, if the glaze can dissolve more then it's a good idea to add it if you are looking for a durable glaze. If you look at the screenshot above in the bottom half of the image you can see the silica is 1.68 so at the bare minimum it is about 1/3 short of where it should be. Overloading a matte glaze can quite often turn it into a gloss glaze  and really oversupplying it can turn it back into a matte and crazed glaze so there are parameters within which to work. 

Another aspect of silica that is often overlooked is the mesh size of the silica being used. When you mix glazes using a 325 mesh size silica more of the silica will likely be dissolved in the glaze than if you use 200 mesh. (not applicable for the SCM)

One more thing is to look at the colouring oxides used. Don't use crazy amounts of colouring/transition oxides, use the least you can to still achieve the effect/colour you are after.

There is a good article on toxicity that includes materials here if it helps.