| Orton Firing Tip: Firing Lead Free Glazes |
Reprinted with the permission
of the Edward Orton Jr. Foundation
Change 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:
- Make test firings of the body and glaze to their recommended
cone number, first the unglazed body can then the glazed bisque.
- 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.
- 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.
- 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:
- Make sure cracks and holes are repaired to keep heat in your
kiln.
- Fire slower during the early part of your firing, before red
heat (below 1200° F). This allows heat to soak into the refractory
and even out temperatures in the kiln.
- 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.
- 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 glaze and body fit, heat distribution and measuring
heatwork 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 |
|
