Tuesday, January 3, 2012

got a glaze question

I've been back into the clay for several days.  I took a break to do the usual Christmas stuff plus, I have to admit, I was putting off the major clean-up that had to happen before my work space could be usable. My first priority was to take care of a couple of commissions- set of mugs, some salt pigs.  Now I'll get on to some of the ideas I've have popping around in my head.
Here's what I'm wondering:  I have some ideas about using this black engobe.  I'm sure it's not food safe to use unglazed, but what about with a clear glaze over it? 
This is it with a coating of clear glaze.  

  Here it is again on a french butter crock lid.

 I've never used this engobe, even under a glaze, on any surface that would come in contact with food, but  would like to if I were sure it was safe.  I've been afraid the engobe might become suspended in the glaze during the firing and be part of the surface.  So can anyone tell me if I need to be concerned about this, or is it safe to use as long as it's under a glaze?
This is the engobe recipe:
Black Engobe 2
Wollastonite              41
Frit 3110                   15
Bentonite                  30
Chrome Oxide            4
Cobalt Oxide              3
Copper Oxide             2
Manganese Dioxide   5
Recipe is copied from an old Ceramics Monthly.  I use it at cone 6 oxidation, but the recipe notes that it can be used in 06-6 oxidation or reduction and that it has an interesting luster in raku. On it's own, no glaze, it has an interesting orange peel texture at cone 6. 

Here's one I haven't tried which was part of the same article.  I think I passed on this one because I thought it might be too blueish under a white glaze:

Black Engobe 
Wollastonite               42
Frit 3110                     15
Bentonite                    30
Cobalt Oxide                3
Red Iron Oxide           10

I should probably test this one to see if I like it.  It's just that I have a fair amount of the other already mixed.

If anyone knows the answer to the food safety question I'd appreciate hearing from you.


  1. I've used similar engobes under clear glazes and the surface has always remained smooth and impermeable. I have never had them tested but I have used them with acidic foods and never noticed any staining. I would try it and see how the surfaces looks/feels then try putting acid food such as strawberries on them see if they stain. If you are still worried definitely get it tested.

  2. Thanks for your imput. I will be dipping with this glaze so I'll get good coverage. I'll try the acid test, but maybe should break down and send a test tile to a lab. Up to now I've relied on glazes that are reported as food safe or not by their publishers.


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