Porcelain Points

How to Fire Porcelain for Consistently Good Results

Reprinted from Creation Palette

WHEN IT COMES to firing porcelain, no two furnaces are exactly alike, but you'd never know it by looking at typical firing instructions.

Many assume that all furnaces produce good results at the same temperatures. They don't--which means porcelain is either under-fired or over-fired--unless you know a few techniques to ensure proper firing no matter what type of furnace you use.

Just a Few Simple Steps
Before you begin, calibrate the furnace to the manufacturer's specifications.

Be sure to stay with one type of firing tray for awhile, since the size, color, and material of the tray can affect the way heat gets to your porcelain and how it fires up.

Then follow these simple steps:

1. Program your furnace for a first dentin bake according to the chart.
2. Fire some porcelain, either on a test crown or on platinum foil.
3. Examine your fired porcelain for surface texture and shine. You should see a light gloss while retaining some texture.
4. Compare the porcelain to the pictures in the Creation Workbook on page 11/19. Pay more attention to shine vs. texture (which depends on how you finish the build-up). Porcelain should look slightly "wet."
5. If the porcelain appears granular or has a matte finish, raise the high temperature of the firing cycle, leaving the heat rate and hold time the same. Repeat Step 3.
6. Once you achieve the right appearance, add the required first-body temperature increase to all other cycles on the chart. In other words, if you have to raise the first dentin by 15°C, then raise both opaques, shoulders, glaze, and so on, by the same 15°.

For a nice natural glaze, most technicians find a half-minute hold at the temperature determined by this method yields a good shine.

If you do a lot of hand polishing, stick with the zero-hold time on the glaze. And if getting a good glaze takes more than a minute of hold time, then the porcelain is probably under-fired.

Ask for Our Helpful Guide

For your convenience, we have tabulated the typical temperature adjustments for some of the most popular furnaces in use.

As long as your adjustments are close to the settings (within 10-15°) you should be fine. If they are way off, however, check your temperature calibration again, then check your rate and hold times with a stopwatch.

Telltale Signs of Improper Firing

• Weak or brittle porcelain that cracks on the bench or intra-orally
• Chipping & flaking that occurs while grinding
• Excessive pitting or porosity in the porcelain
• Porcelain shoulders that never seal the margins, even after numerous applications
• Lack of chroma
• Excessive value
• Lack of translucency
• Overall "lifeless" look to porcelain

If you'd like a copy of our temperature guide, call your Jensen Representative at 800-243-2000.

Revision 0, 3/19/99