To eliminate lead, glazes have to be reformulated. This can change some of their properties. This leads to shivering or crazing of the glaze. Sometimes bodies will have to be bisqued to a higher or lower cone number to solve a fit problem. Lead softens a glaze and allows it to be fired over a broader range.
Glazes made without lead have a narrower firing range.
Use witness cones placed near the ware to be sure the proper cone number was reached. Differences may exist between the Kiln sitter and witness cone or between locations in the kiln. Firing with a controller to a cone number or a temperature may not be exact. If shivering occurs, fire one cone cooler. You may have to select another body or a different glaze.