Arnold Howard on fri 6 aug 99
Hi Linda,
Thanks for you very interesting letter about leaded glazes.
I would appreciate it if you would read the following article, which is
to appear in a newsletter. If you see any mistakes, please let me know.
Arnold
Firing Lead-Free Glazes
By Arnold Howard
Firing seems more difficult in the 90's, because glazes contain fewer
heavy metals, such as lead.
Lead makes glazes flow smoothly at high temperature, leaving a
beautiful sheen. Glazes of the past could tolerate poorly vented or
underfired bisque. The leaded glaze expanded and contracted with the
underfired body. Now even leaded glazes have so little lead that they
are more sensitive to crazing, or glaze cracking.
On the positive side, lead-free glazes also offer benefits. Leaded
glazes flow freely at high temperature, whereas lead-free glazes are
stiffer. Apply lead-free glazes of different colors right next to each
other. Since they don't flow when hot, gravity won't make them run
together. You can also layer lead-free glazes. Paint a pink flower onto
a white background, for instance.
Lead-free glazes won't drip onto the kiln shelf as readily as leaded
glazes. Stilts are easier to remove from the ware. Christmas ornaments,
eggs, or other difficult shapes are easier to stilt, because the glaze
doesn't run as much.
"Lead-free glazes are tricky," says Merle Peratis of Capital Ceramics.
"They're not as forgiving as leaded glazes. But if people will take the
time to learn to fire them correctly, the results will be just as good
as with leaded glazes."
@SUB SUB HEAD = Application Basics
1. Don't apply the glazes too thick.
2. Lead-free glazes must dry naturally without force-drying. A cool
breeze or a fan is okay, but avoid using heat lamps or hair dryers or a
kiln to dry the glaze.
3. Dry the glaze completely between each coat. When it is dry, the
glaze will look dull and will feel warm, not clammy. Lead-free glazes
should dry 24 hours before firing.
@SUB SUB HEAD = What Type of Kiln?
Lead-free glazes are easier to fire in an electronic kiln. One reason
is that controllers can hold, or maintain, the firing temperature at
the end of the firing. You needn't replace your kiln to fire lead-free
glazes, though. Switch-operated kilns work well, too.
@SUB SUB HEAD = Pyrometric Cones
"When you ask people about the shelf cones in their kiln, they come
back with, `What's a shelf cone?'" says Frank Kahanic of Ceramichrome,
Inc.
"A big problem is that people are not using witness cones, and never
have," says Ann Alvarez at Duncan Enterprises.
Yet pyrometric cones on the shelf are essential in firing lead-free
glazes, even in electronic kilns. To save on the cost of shelf
(witness) cones is gambling with a whole load of ware. Without shelf
cones, you will never know if the Kiln Sitter or electronic controller
is out of adjustment.
Pyrometric cones are small pyramids of clay and fluxes that soften and
bend when heated to maturity. The small 1<$E1/8>" cones are mounted in
the Kiln Sitter. Large 2<$E1/2>" cones are placed on the kiln shelf
near the ware. Self-supporting large cones are easier to use than the
original cones that mounted in a clay or wire holder. The correct slant
is built into the base of the self-supporting cone.
Place three cones on each shelf: the firing cone, and the next cooler
and hotter cone. For instance, if you are firing to cone 06, place 05,
06 and 07 cones on each shelf. Keep them at least 3" from a peephole to
avoid cool air. The shelf should be kiln-washed, or over-fired cones
will stick to it.
As your firings become consistent, you may decide to use only the shelf
cone you are firing to instead of the three cone system. But even with
consistent firings, place a cone on every shelf.
Place extra cones near heavy masses of ware, such as thick plates. This
will indicate whether the ware received enough heat. Thicker ware
requires more heat. This is why heavy pieces are often underfired.
@SUB SUB HEAD = Even Heat Distribution
Cones are placed on every shelf to check how evenly your kiln fires.
Even heating is important in firing lead-free glazes.
Moving ceramic mass inside the kiln changes heat distribution. The
greater the mass of ware and furniture, the more heat is absorbed.
Therefore, distribute the ware evenly throughout the firing chamber.
What can you do if your kiln fires unevenly? Distribute the ware inside
the kiln to compensate for it. Place the heavier pieces where the kiln
fires hotter. They will absorb more heat. Place lighter or fewer pieces
where the kiln fires cooler.
For instance, if the kiln fires hotter in the center section, place the
heaviest pieces there. If the bottom fires cooler, place lighter pieces
on the bottom. By checking the shelf cones after each firing, you will
soon learn to load your kiln for even heat distribution.
@SUB SUB HEAD = Venting the Kiln
As the ware is heated, impurities in the clay are pushed to the clay
surface and burned. Without enough oxygen flowing through the kiln,
though, impurities remain trapped inside the ware.
When bisque ware is heated during the glaze firing, remaining
impurities in the clay will surface as a gas. The escaping gas passes
through the glaze, leaving small ruptures, such as pinholes and
craters.
Since leaded glazes flow, the small ruptures in the leaded glaze
usually smooth out and disappear. Lead-free glazes are stiffer, so
escaping gases leave craters in the glaze. This is why greenware
destined for lead-free glazes must be well-vented. Impurities must be
burned off in the greenware firing.
To vent the kiln, leave the peephole plugs out and raise the lid to the
venting position for the first hour of firing. Keep the switches at a
low setting during venting. Lower the lid and insert peephole plugs
only after firing odors disappear. You can also hold a mirror above the
peephole for a couple of seconds. (The mirror should be at room
temperature, not hot.) If it fogs, moisture is still escaping, and the
kiln should remain vented. The heavier the load of ware, the longer the
kiln should remain vented.
If the kiln is fired too fast during the venting period, impurities
remain locked inside the piece. If you break the piece, you can
sometimes see the impurities as a black line. Examine the clay at the
break. The impurities will look like a pencil line running along the
center where the inside of the clay is exposed.
If the bisque turns out gray instead of white, it is because the carbon
pushed to the surface, but because of lack of oxygen, it didn't burn
off. Refire the piece with better venting before applying glaze.
The down-draft kiln vent pulls oxygen through the firing chamber
throughout firing. When using the down-draft vent, leave the peephole
plugs inserted and the lid fully closed during the entire firing. No
further venting is needed. The down-draft kiln vent also helps even out
heat distribution.
@SUB SUB HEAD = Firing Time
The trend in the 90's is to fire faster. The heavier the load, however,
the slower the kiln should fire. Otherwise the heat won't penetrate the
ware completely. "It takes longer to cook six potatoes in a microwave
than it does to cook one," explains Frank Kahanic. "It's the same with
kilns."
@SUB SUB HEAD = The Greenware Firing
"Nine out of 10 times," says Jean LeBlanc of Duncan Enterprises, "the
problem with lead-free glazes is underfired bisque."
Firing experts all seem to agree. Chuck Trott, a ceramics engineer at
Mayco Colors, Inc., says, "I see more lead-free failure because of
improper greenware than anything else in the industry."
"If bisque is underfired when you apply lead-free glaze," says Ann
Alvarez, "it is likely to cause crazing, fine cracking in the glaze.
This is the most common problem we see."
"You cannot have even a slight underfiring, or you will have problems
with lead-free glazes," adds Frank Kahanic.
Experts also agree that the greenware should be fired two cones hotter
than the glazed ware. Firing an extra cone hotter adds very little to
electrical cost and makes the bisque harder. It rings when you thump it
with your finger. It becomes less porous and more water resistant.
Glass forms in the clay, and the clay shrinks, when it has been fired
to maturity. If the clay does not vitrify, or reach maturity, it
changes size in daily use with changes in temperature. Lead-free glazes
require a fully matured clay body to avoid crazing.
@SUB SUB HEAD = The Glaze Firing
Bruce Locke at Hobby Colorobbia recommends a 10 to 15 minute hold time
after the end of the glaze firing. (Hold means maintaining the firing
temperature.) The 10 to 15 minute hold helps the temperature become
even throughout the kiln. It also gives the glaze more time to dissolve
bubbles.
The hold should be started 10 deg. F. below the maturing temperature.
The easiest way to do this is with an electronic kiln. Program the kiln
to underfire the cone by 10 deg. F. and then to hold the temperature
for 10 to 15 minutes. Do not allow the firing cone on the shelf to
overfire. (See your kiln instructions for more details.)
Glaze firings should be of medium speed rather than fast. Firing
latitude on most lead-free glazes is 1 <$E1/2> cones.
If your kiln is not heating evenly, load the glazed pieces in the same
shelf positions they were loaded during the greenware firing. Thus all
the greenware pieces will fire two cones hotter than the glaze firing
even though the kiln fired unevenly.
Cooling the glazed ware too quickly can cause crazing. Some people
remove the ware with asbestos gloves. This is never advisable. Keep the
lid closed until you can remove the ware bare-handed after it has
cooled to room temperature.
@SUB SUB HEAD = Test Firing
Whenever trying new lead-free glazes or clays, test a sample piece
before firing a full load. Fire a sample with a load of regular ware.
Or use a small test kiln.
Testing is important, because the "fit" between lead-free glazes and
the clay body is more critical than with leaded glazes. "Fit" means the
glaze and clay expand the same. "If the glaze and the body shrink and
expand differently," says Mike McGuire of Mr. & Mrs. of Dallas,
"something's going to pop, because the glaze is after all basically a
glass compound, and glass is not extremely expandable."
Poor fit between the glaze and clay body can result in delayed crazing
after the piece has been cleaned in the dishwasher a few times. Delayed
crazing from poor fit between glaze and clay body is often likened to a
man with a 36" waist zipping up a pair of 32" pants. He may be able to
get into the pants, but if he bends over, he'll probably tear out the
zipper. Though delayed crazing is most apparent sometime after firing,
it is usually visible to the trained eye when the ware is removed from
the kiln.
"Because of the popularity of stains over the years, people have become
lax in their firing skills," says Frank Kahanic. "They don't calibrate
their kilns, because they don't have to. It could be 03 to 08 and the
stain would still work."
This is why lead-free glazes are tricky to fire. But as we gain
experience, lead-free glazes will be as easy to fire as any other
glazes. Lead-free glazes do not demand new skills; just the same firing
basics that kiln manufacturers have always recommended.
@BODYTEXT,LEFT = ____________
Many thanks to our friends in the ceramics industry who shared their
expertise on lead-free glazes.
This article first appeared in Popular Ceramics (800-331-0038).
--- Linda Arbuckle wrote:
> ----------------------------Original
> message----------------------------
> > From: Arnold Howard
> >
> > It is a common belief, in hobby ceramics, that
> lead in glaze helps the
> > glaze to fit the clay body. Therefore, unleaded
> glazes craze more than
> > the older leaded glazes. I don't see how lead
> could help with
> > glaze/clay fit. What is your opinion?
> >
> > Arnold Howard
> >
>
> Historically the choice was between lead glazes and
> alkaline glazes for
> lowfire. Thermal expansion of lead is moderate,
> while thermal expansion
> of sodium and potassium (alkaline fluxes that melt
> well at low-fire
> temperatures) is high. This means sodium and
> potassium expand a
> relatively large amount upon heating and contract a
> similar amount on
> cooling. If the glaze shrinks more than the body,
> you get crazing.
> Alkaline blue glazes are an example of a lowfire
> transparent glaze that
> is unleaded (and it crazes from high
> sodium/potassium).
>
> Fortunately, many glaze materials form eutectics,
> melting lower in
> specific combinations, so that lowfire glazes are
> not limited to only
> those materials that melt by themselves at lowfire
> temperatures. Boron
> is another material that works as both a flux and
> viscosity agent at
> lowfire temperatures.
>
> Another big problem with crazing of lowfire glazes
> is delayed crazing.
> Lowfire bodies don't usually vitrify (i.e. become
> fused enough to hold
> water w/o a glaze as some stoneware and porcelains
> do), and the body can
> absorb water thru the foot, swell slightly, and
> cause delayed crazing.
> Using terra sigillata on the foot helps reduce
> porosity and counter this
> problem.
>
> Firing a bit higher, say to 03, also helps, as you
> just reach the
> temperature where cristoballite is formed from free
> silica in materials.
> Cristoballite inverts (becomes about 2% smaller)
> around the temperature
> paper burns, 451 F. A bit of cristoballite is good
> for lowfire as that
> inversion in the clay body puts a glaze under a bit
> of compression and
> counters crazing. Too much cristoballite shrinkage
> of the body (more of
> a problem for high-fire potters), and you get
> dunting, (the piece cracks
> in cooling) and maybe shivering (the glaze pops off
> the pot in cooling).
>
> It's difficult to make a lowfire glaze that will
> NEVER craze, but it's
> possible to cut down on the problem quite a bit.
> Thermal shock
> aggravates crazing (e.g. making iced coffee by
> pouring hot coffee over
> ice cubes in the cup), as does dishwasher-ing and
> microwaving. I do
> microwave tea in my terracotta cups. They don't seem
> to heat up any more
> than some stoneware cups I have.
>
> Back to the lead issue.... lead and the alkaline
> fluxes (potassium,
> sodium, lithium) make a soft glaze that is not very
> durable. If you look
> at old lead or alkaline glazes that have been
> exposed to weathering,
> they begin to break down. Adding alkaline earth
> fluxes like calcium,
> magnesium, or strontium (barium, too, but it's
> toxic) contribute to a
> stronger glaze. Issue is that these would also
> stiffen up the glaze
> some, and probably make is more toward frosty if
> thick than transparent
> like lead or alkaline glazes. The majolica glaze I
> use is high in Ca for
> a lowfire glaze, very durable, fired to 04-03, lead
> free.
>
>
> --
> Linda Arbuckle
> Graduate Coordinator, Assoc. Prof.
> Univ of FL
> School of Art and Art History
> P.O. Box 115801, Gainesville, FL 32611-5801
> (352) 392-0201 x 219
> e-mail: arbuck@ufl.edu
>
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