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COLUMBIA

Glaze Colour Continued from Page 6

usually fragile. It is easy to predict consistency problems
for such mechanisms. Potters can fiddle with reactive
glazes, but industry generally stays away from them.

Do the results depend on a fragile melting mechanism?

Is vigorous melting (and running) required to develop
the colour and character? As noted above, such glazes
may not only be prone to colour problems, but also
running and blistering. Glossy rutile-blues are an
example. Another thing to remember is that certain raw
colours and stains volatilize (vaporize) above certain
temperatures.

Kiln atmosphere, ramp.

‘The mechanism of colour development in a glaze may
depend on kiln atmosphere (i.e. strong reduction,
weak reduction, strong oxidation), or on the speed or
curve of both the ramp up and down. Your kiln may
have variations in the atmosphere or your electric kiln
might be firing near reduction because of poor airflow
combined with carbon burn-of.

Has it been put on the right side of the glaze layer?

‘The same metal oxide will develop different colours
depending or whether it is painted under or over a glaze.
If it is painted under, for example, glaze thickness, bubble
population, crystal development and chemical inter-
action between glaze and colour will

only if they are applied thickly enough. Others develop
the desired effect when they are thin and the underlying
body imposes some colour. Light-coloured clay bodies
foster the development of bright colours, iron bearing
bodies subdue colours (especially when the glaze is thin.)
Many glazes will develop colour of different character on
refractory porous bodies compared to vitreous ones.

Conclusion

‘Try taking a cheap microscope and have a really close
look at your glaze surface. You might be surprised at
how much you learn about why the glaze looks the way
it does. Understanding the mechanism of the colour and
surface will help you understand how to trouble-shoot
problems. It does not take rocket science, anyone can
note the transparency, micro-bubbles, crystalization,
variegation in colour and surface (phase differences), etc.
And do not shy away from chemistry, in many cases you
just need to know if an oxide is present or not and how
much is there. Search for ‘ceramic chemistry’ on Google,
download a free trial of INSIGHT and work through
the lessons section of the manual to learn how to enter a
recipe and see its formula and analysis.

More technical articles are available online at:
http://ceramic-materials.com/cermat/education/index.html

http://digitalfire.com/

shape the effect.

Would a stain be better?

Achieving and maintaining an

exact shade of colour can be quite
difficult with raw colouring oxides,
especially if a blend is being used.
For example, many people use
cobalt, iron and manganese for
black. However colour shifts are
common with this approach and

it is usually not obvious which
metal oxide should be increased

or reduced to stabilize the colour.
Stain companies have invested
considerable time to develop colours
that are reliable and stable (often
containing zircon, alumina, silica in
addition to the metal oxides.) Stains
are more expensive, but the stain
company assumes a burden that is

Greenbarn's catalogue is
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In an effort to be move enwicormnen:

www.geeenbarn.com

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use of |
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often difficult for most companies or

potters to handle. 9548 192 Street,

Is the glaze the right thickness? On the
right body?
Many glazes develop deep colour

Surrey, B.C. V4N 3R9
Phone: 604.888.3411
Fax: 604.888.4247

Tuesday-Friday 9-5
Saturday 9-1

Closed Long Weekends
greenbam@telus.net

Ween Darn

POTTERS SUPPLY LTD.

Potters Guild of BC Newsletter - June 2008