Orton Foundation Firing Tips: Firing Lead Free Glazes

Changes in glazes

Lead free glazes are becoming the standard for personal and 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 that do not contain lead.

To eliminate lead, glazes have to be 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

· Brush marks may show after firing

· Narrow firing range

· May not be compatible with as many clay bodies. This leads to shivering or crazing of the glaze.

· Color does not match lead glazes

For problem-free results with lead free glazes, firings must be more closely controlled and kilns well vented. Sometimes bodies will have to be bisqued to a higher or lower cone number to solve a fit problem.

Why Do Problems Occur?

Lead softens a glaze and allows it to be fired over a broader range. Glazes made without lead have a narrower firing range.

Typically, lead glazes are able to be fired over a four cone number range (example: 06-02).

Lead free glazes typically have a range of less than two cone numbers (example: 06-05)-less than half of that of 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 the glaze can crack (crazing), or it can be pushed together on to itself to a point where shivering occurs.

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 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.

3. If crazing occurs and the witness cone indicates the glaze and bisque firings are properly fired, make some tests firing the bisque progressively hotter (e.g. if you bisque to 05, test to 04, then 03).

4. If shivering occurs, fire one cone cooler. You may have to select another body or a different glaze. 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 Temperature Uniform?

If the 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 in another part 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 1200°F). 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 most temperature variations in half. Hoods above the kiln will not help temperature uniformity problems.

Firing Lusters

What Are Lusters?

Lusters are metal-based transparent overglazes available in rainbow colors. They are applied over gloss, satin and matte glazes as well as porcelain bisque.

Beautiful if applied over china paints, lusters can also be applied over other lusters. When fired they are iridescent.

Lusters can be used to save streaky or unattractive underglazes or to give a new or different look to a piece. The most popular and perhaps best known luster is Mother of Pearl which has vari-colored appearance after firing.

How Do Lusters Work?

· Metallic salts, colorings, a reducing agent (usually organic binder such as oil of lavender) and resin are mixed with oil medium

· Lusters are applied to glazed surface with brush

· During firing, the reducing agent burns out causing localized reduction

· The oil medium and resin burn out and leave the metal

· A thin layer of pure metal is deposited on glaze

Concerns When Using Lusters

1. Dust on ware before application causes uneven, splotchy look:

· Area should be dust free

· Clean ware with alcohol before application and let dry

2. Essence brush cleaner left on brush causes brush to pick up luster and leaves brush strokes:

· Let brushes air dry 2-3 hours after cleaning with essence

3. Wet brushes used for lusters causes streaky look:

· Let brushes air dry 2-3 hours after cleaning with essence

4. Using same brush for different colors of luster causes muddy appearance:

· Maintain and use separate brushes for each color

5. Too much luster causes:

· Powdery appearance

· Open/dull spaces between glossy luster areas

· Blistering or crazing

· Brush marks on luster

· Apply 2 even, thin coatings

6. Too little luster causes uneven appearance:

· Apply 2 even, thin coatings

7. Piece fired before luster dried causes color to disappear or form white splotches:

· Let dry overnight before firing

8. Over firing of ware causes weak colors:

· Use witness cones

9. Under firing causes lusters to powder off:

· Use witness cones

10. Insufficient ventilation causes migration, contamination or a frosty appearance after firing:

· Insure ventilation in kiln

Reprinted with permission form Orton Foundation

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