Mason stain food safety in clear glaze

[NanS]

Beginner question: Is there a rule of thumb about food safety when adding mason stains as a colorant to a clear base glaze?  I'm wanting to experiment with adding a bit of mason stain to a couple of clear glazes that I've mixed up, to create a transparent celadon-like look. The percentages will be very low, 3% or less.    I have been assuming that these glazes would be food safe, but it occurred to me to double check. I've been trying to research this on this forum and online but haven't come up with a definitive answer.  (The colors I'm looking at are right now are Robins Egg Blue 6376, Chrome-Tin Violet 6304, Vanadium Yellow 6404, also Copper Carbonate.).  I'm wondering if this is a case by case basis depending on the colorant, or if there is a rule of thumb that the resulting glazes would be considered food safe. Thanks people in advance for any light you can shed on this question.

[Min]

Ceramic stains are basically colouring oxides that have been fritted into a safer form for the potter to use and widen the range of colours available. If you have a well balanced stable base glaze then there is a good chance the glaze coloured with stains will be stable if the glaze is well melted and the amount of stain used is within reasonable amounts. Having plenty of silica and alumina in the base glaze, not overloading the boron and firing to maturity all help in creating a durable base glaze. BTW if you are going for a celadon like look you probably won't need 3%. 

Robins Egg Blue and Vanadium Yellow both contain vanadium which is quite toxic in the raw form so using a stain to supply it is by far the safer route. If your base glaze doesn't leach with base and acid testing then it's probably okay but the only way to know for sure would be to get it lab tested. I don't know of many people who actually do this. 

[kswan]

You will also want to be sure that your base glaze doesn't have zinc in it if you plan to use the Chrome Tin Violet. Zinc makes chrome turn brown. 

 

[Callie Beller Diesel]

The reason you don’t find definitive answers about whether or not a specific material is or is not food safe is that it depends on what else it’s mixed with, and in what proportion. The specific firing conditions can also play a part. Each combination of materials is a little different, and that affects the properties of the end of the result. The absence or presence of any one material in a recipe does by itself not indicate whether or not a glaze is a good one to use on dishes for food service. To give more specific answers, we have to get into chemistry numbers and parameters. We can do that here, and will happily if you want, but I like to check first before infodumping. 

Food safety is also a bit of a misnomer. Most people come into it thinking that it means things aren’t going to leach out of your glaze and poison you off. That happens a lot less frequently than you might think. The greater concern when making food ware is durability. You don’t want your glaze wearing off in the dishwasher, you don’t want it crazing or scratching easily, you don’t want it staining or to be hard to clean, and you definitely don’t want it to change colour or texture over time. If the durability requirements are met, the likelihood of leaching falls drastically.

 

 

[NanS]

Thanks very much Min, Callie and kswan.  My glaze is the Glossy Clear Base from the Mastering Cone 6 book, and is zinc free (https://glazy.org/recipes/134907). I'm firing to cone 6 in an electric kiln. Your feedback in useful, and as with most things in ceramics I can see that the answer is not straightforward!  Also, thanks Min for the tip on using less than 3%, I think I'm planning to aim for 2% for my first tests. 

 

Thanks also Callie for the useful thoughts about food safety.  Much appreciated!

[Min]

1 hour ago, NanS said:

I think I'm planning to aim for 2% for my first tests. 

To save yourself some work I would suggest making up 200 grams of base glaze with no stain as a baseline and dip a small test tile then add 0.60 stain (0.30%), re sieve then dip a second test tile, keep repeating this until you get up to 1.80% (that would be 6 additions of 0.60 grams of stain). Given that the 200 grams will be lower with each dip this isn't super accurate but it will get you in the ball park. 200 grams will get you the baseline plus 6 test of increasing stain. Doesn't have to be increments of 0.30%, if you want to test higher amounts bump it up to 0.50% or whatever you think best. 

A method to divide up the stain quickly is to measure out the total you will be testing, say 1.8% which for a 200 gram base will be 3.60 grams for my example above. Take the 3.60 grams and put it onto a flat smooth surface like a piece of glass or smooth countertop. Now use a metal rib and compress the stain into a flat square shape. With the rib now divide it down the middle then cut each half in thirds, makes 6 piles of stain, each 0.60 grams. Sounds confusing when I write this but it's quicker than weighing out each batch of stain and will get you close enough for a first run of tests. 

 

 

 

[oldlady]

thanks, min for the division lesson!    as always, you are top of my list when it comes to glazes.  wanna do a glaze workshop together over zoom?   our guild has lots of new people stuck on bottles instead of mixing ingredients.   you be the puppet master and i will do exactly what you say.

 

[NanS]

Thanks Min, that's a fabulous plan, and I totally followed along with your instructions.  Super helpful! 

[Min]

1 hour ago, oldlady said:

thanks, min for the division lesson!    as always, you are top of my list when it comes to glazes.  wanna do a glaze workshop together over zoom?   our guild has lots of new people stuck on bottles instead of mixing ingredients.   you be the puppet master and i will do exactly what you say.

 

Everyone here has something to contribute, I'm just one of many but thank you for your kind words. Re teaching a zoom glaze workshop, sorry but this isn't something I'm in a position to do, thanks for the thought though!

3 hours ago, NanS said:

My glaze is the Glossy Clear Base from the Mastering Cone 6 book, and is zinc free (https://glazy.org/recipes/134907).

The chrome tin stain you are thinking of testing probably won't work with this base, likely to get a washed out blue gray colour. Base glaze needs to have at least 10% but better with closer to 20% calcium for any of the chrome tin stains to work, this includes the pinks, reds and purples. 

[Callie Beller Diesel]

Magnesium also will mess with stains that are killed by zinc. It only gets an offhand mention on the Mason reference guide, but there’s a few spots on Digitalfire that Tony Hansen talks about it in passing as well. 

[NanS]

Thanks for the tip to avoid the chrome tin.  I have a couple of other glossy clear glazes I've been experimenting with, I'll check them in Glazy for calcium content to see if they'd work.  Also will look into the magnesium issue mentioned by Callie.  Much appreciate the advice you all have provided.

[Jeff Longtin]

Mason Stains - Ceramic Supply Pittsburgh

Mason makes a Color Chart and it lists the recommended percentages for each stain. The only place I've found it is provided above. It's quite useful. It also lists the recommended amount of zircopax to use.