Lead free or unleaded glazes are now the standard for commercial use. This is due to government regulations and health concerns by the manufacturers.
As their 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 might notice include:
1) glaze does not flow or run as much in firing as lead containing glazes.
2) brush marks may show after firing
3) not as wide a firing range
4) may not be as compatible with as many clay bodies. Can lead to shivering or crazing of the glaze.
5) color does not match lead glazes
6) more surface defects
For problem-free results with lead free glazes, firings must be more closely controlled and well vented. Bodies may have to be bisque fired to a higher or lower cone number to solve problems.
Lead softens glaze and allows it to be fired using several cone numbers. Glazes without lead have a narrower firing range. Example: A leaded glaze may be able to be fired anywhere from 05-08. Lead free glazes may only have a firing range of 05-06.
Since the glaze and the clay body on which it is fired 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, also called crazing. It can also be pushed together on to itself to a point where shivering or crawling occurs, sometimes revealing the bisque underneath.
When using lead free glazes:
1) Make test firings of the body and glaze to their recommended cone number, first the unglazed body and then the glazed bisque.
2) Use witness cones 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 may also be inadequate.
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, to 03 instead of 04, for example. Also see out tips on carbon burnout.
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 can cause problems during the use of the piece.
If temperature in your kiln varies by more than 1-2 cones, then glazed ware in one part of your kiln may fire okay, while ware in another part may have a problem.
Be sure to read the tips page on firing uniformity.