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Firing Lead Free Glazes
Orton Firing Tip -
FIRING LEAD FREE GLAZES
Reprinted with the permission of the Edward Orton Jr. Foundation
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Changes In Glazes
Lead free glazes are becoming the standard for commercial use. This is
due to government regulation and health concerns by the manufacturers.
As the name implies, lead free glazes are made from compositions or materials where lead has not been added.
To eliminate lead, glazes are reformulated. This can change some of
their properties. Some of the differences you may notice include:
* does not flow or run as much in firing as lead containing glazes
* brush marks may show after firing
* not as wide a firing range
* may not be compatible with as many bodies (improper fit). This leads to shivering or crazing of the glaze.
* colour does not match lead glazes
* more surface defects
For problem-free results with lead free glazes, firings must be more
closely controlled and kilns well vented. Bodies may have to be bisqued
to a higher or lower cone number to solve a problem.
Why do problems occur?
Lead softens a glaze and allows it to be fired over several cone
numbers. Glazes made without lead have a narrower firing range.
Typically, lead glazes are able to be fired over a four cone number
range (example 08 to 05).
Lead free glazes typically need to be fired within two cone numbers
(example 06 to 05) - less than half of that for lead glazes.
Glaze and body fit?
Since the glaze and the body on which it is fired (bisque) are made
from different materials, it is important that they expand and shrink a
like amount when heated and cooled. If they don't, then the fired glaze
can be stretched to the point where it can crack (crazing), or it can
be pushed together on to itself to a point where shivering or crawling
occurs.
When using lead free glazes:
1. Make test firings of the body and glaze to their recommended cone
number, first the unglazed body can then the glazed bisque.
2. Use witness cones placed near the ware to be sure the proper cone
number was reached. Differences may exist between the Kiln-Sitter and a
witness cone or from the top to the bottom of the kiln. Firing with a
controller to a cone number or a temperature may not be adequate.
3. If crazing occurs and the witness cone indicates the glaze and
bisque firings are properly fired, make some tests by firing the bisque
progressively hotter (e.g. if you fire to bisque 05, test to 04, then
03).
When you fire hotter, the expansion of the bisque is changed and glaze on the bisque may fit better.
4. If shivering occurs, fire one cone cooler. You may need to select
another body for your bisque. Firing too cool is not a good idea since
the strength is reduced and porosity increased, both of which may cause
problems during use of the final piece.
Is Your Kiln Uniform in Temperature?
If temperature in your kiln varies by more than 1 to 2 cones, then
glazed ware in one part of your kiln may fire okay, while ware fired in
another part of your kiln will have a problem.
Most kilns vary in temperature from top to bottom. To determine how
much your kiln varies, place witness cones on each shelf when making
firings. Usually, there is less difference top to bottom for hotter
firings.
Each kiln has its own personality and the solution for improving temperature uniformity may vary.
If you have glaze firing problems because of too much variation, then we recommend the following:
1. Make sure cracks and holes are repaired to keep heat in your kiln.
2. Fire slower during the early part of your firing, before red heat (below 1200F).
This allows heat to soak into the refractory and even out temperatures in the kiln.
3. Consider changing the switching pattern to even out top and bottom temperatures.
Switch the bottom to a higher setting before the top or vice versa. Higher settings
add more heat.
4. Consider adding an Orton KilnVent. These pull hot gases from the top to the
bottom of the kiln and cut temperature variations in half. Hoods above the kiln will
not help temperature uniformity problems.
Want to learn more?
Read more about Firing Reds in the Orton Firing Line and Technical Tips
publications. Each issue is packed full of articles to help you learn
more about firing. Members of the Orton Firing Institute receive these
publications at no charge. Single copies are available to non-members
at a per issue rate. Orton's 80 minute video, Key Principles of
Successful Firing, is also an excellent resource on firing.
For information on Orton products, see your Orton dealer or
distributor. For information on the Firing Institute or publications,
contact:
Orton Firing Institute
PO Box 460
Westerville, OH 43086
(614) 895-2663 |
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