How to Fire Porcelain for Consistently
Good Results
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:
- Program your furnace for a first dentin bake according
to the chart.
- Fire some porcelain, either on a test crown or
on platinum foil.
- Examine your fired porcelain for surface texture
and shine. You should see a light gloss while retaining
some texture.
- 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."
- 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.
- 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