Help finding out how this glaze effect is created

[lobashpottery]

Does anyone have any idea on how the color variation/movement in the green glaze is created? It is a premade glaze from my local pottery shop, and the employees were unable to tell me how it is done. I have a basic ingredients list, but it does not give specifics on the type of feldspar or frits used. Any help would be appreciated, as I'm new to glaze making.  Thank you!

Here is a link to the basic ingredients sheet. The glaze is called Metallic Green, under the HG (high gloss) section. 

https://www.mnclay.com/msds/glazes.aspx

Edited July 25, 2022 by lobashpottery

[Russ]

My guess is that its rutile.

[Min]

@lobashpottery, are you using this glaze on surfaces that come in contact with food? Reason I ask is when looking at the Safety Data Sheet for that glaze I had some alarm bells go off.

The tiny print I highlighted in blue is a lead biscilicate frit. To get the mottled effect they have used titanium dioxide which is in rutile so that would work too. 

https://www.chemicalbook.com/ChemicalProductProperty_EN_CB8225469.htm#:~:text=LEAD BISILICATE | 65997-18-4

[Bill Kielb]

I’m sorry, I don’t have a specific answer, but maybe this will help:
It likely contains rutile and at least for the top one, it is applied very thick to encourage gravity to make it flow while firing. The glaze in that picture is also crazed which is most often viewed as a flaw for functional wares. My reading of the website says, this is a mid fire glaze which generally means cone 5/6. Without going into the nuances of glaze chemistry, you might want to search Glazy.org for specific colors and cones. Comparing similar glazes often can be a nice way to get a feel for the effect of colorants and components in the glaze such as rutile.

Glaze combinations are amazing and as you progress you will be amazed by the knowledge acquired. Know your cone and search Glazy.org by color and or texture, I think it may help get you started.

The MSDS sheets on that site are not the glaze recipe, they only report material safety data. Glazes in the US are regulated for lead and cadmium. If it contains either, then it will not be considered food safe.  It’s often hard to get recipes for commercial glazes. As you find recipes that you like, you will need to test them which affords the opportunity to adjust their color and characteristics and make sure they fit your clay without crazing like in the first picture.

Anyway, my best idea to start the learning process. There are many glaze formulations out in the public. John Britt, digital fire.com among the many that you can learn from.

Edited July 25, 2022 by Bill Kielb

[Callie Beller Diesel]

It’s also worth noting that the glaze is on an iron bearing clay of some kind. That will have an effect on how the glaze looks, and applying it to a whiter clay would maybe not have as much visual depth.

[lobashpottery]

@Min I have used this glaze on food surfaces. I just checked the website of the pottery supply store which i purchased it from, and it is listed as being leadless. However, I will contact them to be certain of its safety. Thank you for letting me know of the possible danger, I appreciate it greatly!

[Min]

Refer them to the SDS sheet you linked.

edit: it would be really good to find out if they are just using that CAS number as a generic frit number or if indeed it is for lead bisilicate.  I had a look at some of the SDS sheets for their other glazes, there are a lot of glazes in their list that contain that CAS frit number.

Edited July 26, 2022 by Min