Alisa and Claus Clausen on thu 6 apr 00
------------------
Hi Ron,
snip..
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 =
glaze.
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
transparency.
Is that usual for colbalt to behave like that in a gloss base?
When you say heavy metals =22floating on the surface=22, does that mean the =
oxide
contrentation
is saturated so that the glaze surface has metallic areas (such as I have =
found
with 3 precent
of colbalt)?
Is there a rule of thumb for what concentrations of oxides, or is only heavy
metals,
that make an otherwise food safe glaze, unsafe? Otherwise, with a given =
receipe
and
precent of additives, can you adivse me on the safety of the colored
transparents?
Thanks and best regards,
Alisa in Denmark
Ron Roy on mon 10 apr 00
Hi Alisa,
There is only one way to tell if a glaze is stable for sure - have it
tested. If you cannot get a recipe - you have no way of telling if a glaze
is stable by looking at it.
The glaze described below - cone 6 clear - is not a cone 6 glaze there is
too much alumina and not enough flux to melt it properly at cone 6 - so I
am not surprised it came out matte - do I have this straight?
Cobalt is a flux at stoneware temps so I would not expect any matting of a
transparent or any gloss glaze unless there is an over saturation - in that
case the cobalt would form a matte skin on the glazes. 1% would not do that
unless there was very little silica in the glaze.
Yes - when the oxides - are in high enough consentrations they will come
out of the solution as the glaze cools and be on the surface. The oribe
type glazes are like that - so much copper that it is on the surface and
can be rubbed off with your finger.
Yes there are upper limits to how much oxide (say colouring) you can put in
a glaze before it becomes too much - it depends on the glaze as well.
Silica starved glazes (poor glass) can keep in less than silica rich
glazes.
Hamer (The Dictionary of Pottery Materials and Techniques) goes into this
in his descriptions of the colouring oxides - It's a great book to have
Alisa. You would look under copper oxide and he would mention some numbers.
I am not an expert in this area so I don't think I should express my
opinions to much.
If you have too much oxide in a glaze - whether it is a hazard to the user
depends on a number of factors. The toxicity of the oxide, the natural
tolerance of the user to that oxide, how much is released etc. Most people
can tolerate iron very well - up to certain limits. I store iron and have
to watch my intake - so I don't want you supplimenting my iron.
Stability of liner glazes is a factor potters need to take into account.
One sure way to discourage customers from wanting more of your work is to
use the kind of glazes that will change colour in use. If the metal is
lying there on the surface you can be sure glazes will change in use.
Let me know If you have more questions - like most potters I am trying to
deal with these factors while learning at the same time.
RR
>----------------------------Original message----------------------------
>snip..
>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 glaze.
>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
>transparency.
>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 oxide
>contrentation
>is saturated so that the glaze surface has metallic areas (such as I have found
>with 3 precent
>of colbalt)?
>
>Is there a rule of thumb for what concentrations of oxides, or is only heavy
>metals,
>that make an otherwise food safe glaze, unsafe? Otherwise, with a given
>receipe
>and
>precent of additives, can you adivse me on the safety of the colored
>transparents?
>
>Thanks and best regards,
>Alisa in Denmark
Ron Roy
93 Pegasus Trail
Scarborough
Ontario, Canada
M1G 3N8
Evenings 416-439-2621
Fax 416-438-7849
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