search  current discussion  categories  safety - misc 

sv: re: food safety/oxides

updated sat 8 apr 00


Deborah B. on fri 7 apr 00

Alisa & Claus,
Cobalt is a very strong oxide. One of the most powerful. Try using only
1/4 of1 percent for a medium blue. 1/2 percent for a strong blue and 1
percent will generallay give you a very dark blue.

With copper oxide 1 percent will give a lighter tint 2-3 % stronger color
and 5% will give a dark or metallic surface.

>From: Alisa and Claus Clausen
>Reply-To: Ceramic Arts Discussion List
>Subject: Sv: Re: food safety/oxides
>Date: Thu, 6 Apr 2000 10:29:59 EDT
>----------------------------Original message----------------------------
>Hi Ron,
>Any glaze that has metalic metal oxides floating on the surface is not
>durable - what metals are there determins the effect on the user.
>This has been a recent question of mine in adding color to a transparent
>I have two different transp. glosses I am testing. (also one beige matt,
>but by
>virtue of it's matt surface, I am not using it for cups and dishes).
>One of the transp. is ready mixed and described as a hard surface good for
>plates, etc. I have no receipe.
>But the other is Marek's receipe,
>cone 6 clear
>Potash feldspar 40,0
>Filint 25,0
>Whiting 20,0
>China clay 12,5
>Bentonite 2,5
>I have colored them with varying additions of oxides
>Black copper, colbalt, rutile, iron and tin.
>Combinations of up to 5 precent of any oxide.
>A surprise for me, that the addition of 1 precent colbalt, completely
>matted the
>Is that usual for colbalt to behave like that in a gloss base?
>When you say heavy metals "floating on the surface", does that mean the
>is saturated so that the glaze surface has metallic areas (such as I have
>with 3 precent
>of colbalt)?
>Is there a rule of thumb for what concentrations of oxides, or is only
>that make an otherwise food safe glaze, unsafe? Otherwise, with a given
>precent of additives, can you adivse me on the safety of the colored
>Thanks and best regards,
>Alisa in Denmark

Get Your Private, Free Email at