mel jacobson on sun 15 apr 07
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>Date: Sun, 15 Apr 2007 07:50:04 -0400
>Reply-To: JohnSankey@ncf.ca
>Sender: Clayart
>From: John Sankey
>Subject: black colorants
>To: CLAYART@LSV.CERAMICS.ORG
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>I want to build a semi-gloss food-safe true black for ^6, to
>go on standard white stoneware. I'd prefer one that gives a
>solid colour when thin, rather than thick.
>
>The primary choices for black gloss additives seem to be:
>
>red iron oxide (typically 10%)
>manganese oxide (up to 4%)
>cobalt oxide (1-2%)
>chrome oxide (1% or less)
>
>Some say that RIO is a frit at ^6, not a colorant.
>
>I'm limited to the standard firings of my group kiln: bisque ^06
>and glaze ^6 - no experiments possible with temperature.
>
>Does anyone have any suggestions as to any of these?
>
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Ivor and Olive Lewis on wed 18 apr 07
Dear Friends,
Selecting and preparing raw materials to create a black glaze is an =
interesting exercise. Success depends upon an understanding of Optical =
Science and the chemical behaviour of materials chosen to fulfil the =
task.
For most people the firing temperature is a given, fixed by the maturity =
temperature of their clay. Firing atmosphere is not always a choice but =
it does determine the way that some of the colourant oxides will behave =
optically.
In the example given by John Sankey, firing is to be at Cone 6 so it is =
a reasonable supposition that this is a neutral or oxidising atmosphere. =
All colourants reflect the hue we observe emanating from their surfaces. =
All of the other hues are adsorbed. Hues can be grouped as =
complementary, vis red/green, blue/orange and yellow/ violet. When these =
pairs are mixed they prevent light, or most of it, from being reflected,
Knowing this fact enables us to select groups of metallic oxides or =
carbonates. So trial with Cobalt oxide and Red Iron oxide, giving blue =
and orange, might be a good place to start.
Line blending will help to get a good balance. If Iron oxide gives a =
greater red response with a selected base glaze add a small proportion =
of Chrome oxide. If it tends towards yellow, Manganese dioxide, which =
can give a purple response would be an appropriate choice.
Fear about excessive melting can be overcome by testing and adjusting a =
base glaze if it runs. My thoughts would be to counteract excess =
fluidity, should it happen, by adding a small percentage of kaolin or =
quartz.
Hope these notes help.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
Edouard Bastarache Inc. on fri 20 apr 07
Hello Ivor,
you had good reasons not to worry about trivalent
chromium
(green chromium oxide) and manganese in your
soup....
I-Trivalent chromium salts are absorbed after
ingestion, but only 1% to 25% of the dose ingested
is absorbed. The extent of absorption varies with
the particular salt ingested and the circumstances
of ingestion.
II-Gastrointestinal absorption is generally low
(5%).
Very few poisonings have occured after ingestion.
Later,
Edouard Bastarache
Le Français Volant
The Flying Frenchman
Sorel-Tracy
Quebec
http://www.sorel-tracy.qc.ca/~edouardb/Welcome.html
http://perso.orange.fr/smart2000/livres.htm
http://www.pshcanada.com/Toxicology.htm
http://www.ceramique.com/cerambooks/rayons/technologie.php
http://www.flickr.com/photos/30058682@N00/
Ron Roy on fri 20 apr 07
What Ivor has left out is durability and toxicity concerns. If the glaze is
to be used next to food then durability is a concern - as is the relative
toxicity of the colouring oxides.
In this case - both manganese and chrome are volatile in out firings - a
concern for the potters health - and also a concern due to leaching into
food.
The better these concerns are addressed the more versatile the glaze will be.
RR
>Dear Friends,
>Selecting and preparing raw materials to create a black glaze is an
>interesting exercise. Success depends upon an understanding of Optical
>Science and the chemical behaviour of materials chosen to fulfil the
>task.
>For most people the firing temperature is a given, fixed by the maturity
>temperature of their clay. Firing atmosphere is not always a choice but it
>does determine the way that some of the colourant oxides will behave
>optically.
>In the example given by John Sankey, firing is to be at Cone 6 so it is a
>reasonable supposition that this is a neutral or oxidising atmosphere.
>All colourants reflect the hue we observe emanating from their surfaces.
>All of the other hues are adsorbed. Hues can be grouped as complementary,
>vis red/green, blue/orange and yellow/ violet. When these pairs are mixed
>they prevent light, or most of it, from being reflected,
>Knowing this fact enables us to select groups of metallic oxides or
>carbonates. So trial with Cobalt oxide and Red Iron oxide, giving blue and
>orange, might be a good place to start.
>Line blending will help to get a good balance. If Iron oxide gives a
>greater red response with a selected base glaze add a small proportion of
>Chrome oxide. If it tends towards yellow, Manganese dioxide, which can
>give a purple response would be an appropriate choice.
>Fear about excessive melting can be overcome by testing and adjusting a
>base glaze if it runs. My thoughts would be to counteract excess fluidity,
>should it happen, by adding a small percentage of kaolin or quartz.
>
>Hope these notes help.
>
>Best regards,
>
>Ivor Lewis.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
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