Specifications

exceeds this point, the clay actually starts to soften or
become liquid-like. Although the overfiring may not be
noticeable to the naked eye, some indications are that the
bisque is hard to glaze, underglazes appear darker or fainter,
or glaze colors seem washed out. The ware will also seem
denser or smaller than normal, due to shrinkage.
Underfiring
The complete opposite of overfiring is underfiring.
Underfiring may not be evident until the ware has been glazed
and used. Greenware or bisque can sound hollow or brittle. A
glaze that performs poorly maybe caused from underfired
greenware or even underfired glaze. Problems that can occur
are: blistering, crazing, pinholes, gray discolorations and
black specks in the ware or glaze surface. Fractured pieces can
be directly associated with underfired bisque.
Bisque Firing
Firing low-fire whiteware greenware to mature the clay is
called a bisque firing. Mayco glazes should be applied to
bisque that is fired to a minimum of one cone hotter than the
glaze, but a good rule to follow is to fire two cone numbers
hotter to assure that the minimum of one cone hotter is
reached. During the bisque firing, the organic material in the
greenware or clay is burned off as gas. If the bisque firing is
not at least two cones hotter than the following glaze fire,
additional gases may be produced and show up as glaze
defects such as pinholes, blistering and/or craters. Utilitarian
items should be fired to a minimum of two cones hotter to
make the ware denser and more durable. Check with your
clay manufacturer to make sure it can withstand these temper-
atures; some clays start to deform at Cone 03 or 02.
Greenware can be safely set on the shelf if no color has
been applied. Generally greenware is not stilted during bisque
firing; if placed on stilts it may soften and deform at the higher
temperatures. Greenware prefers to be fired as it naturally sits.
Cups and pieces with handles or protruding extensions may
distort during a higher bisque firing (cone 02 or higher).
Pieces that are thin at the top and heavy at the bottom, (i.e. a
heavy footed bowl) may fire better inverted upside down.
Porcelain and stoneware are never stilted in any firing.
Opaque and translucent underglazes are applied to
greenware and mature at the same temperature as the
bisque. Underglazes are clay with color pigments added
that are formulated to shrink and develop during the bisque
firing temperatures. If an underglaze seems not dark or rich
enough, it may be underfired. Underglazes that seem dark
may be overfired. Applying underglazes on bisque can lead
to disappointing results, such as faded color or as a defect
called "shivering," where the underglaze has not shrunk at
the same ratio as the ware.
Glaze Firing
All glazes all have a maturation point to which they should
be fired. This is stated on each jar on the label. The important
thing to remember is that a glaze firing is a minimum of one
cone lower (cooler) than the bisque firing temperature.
However, a two cone difference is recommended to avoid glaze
defects, as discussed above. Glazes that are labeled dinnerware
safe are only dinnerware safe when the shelf cone stated in the
instructions has been reached. Glazes that are underfired may
appear cloudy or milky and lack the proper sheen to the surface.
Sometimes glazes can be manipulated to accomplish a specific
effect by firing to different temperatures.
All glazed objects need to be stilted when possible,
excluding stoneware and porcelain. Stilts should be fitted to the
size of the object being stilted. Undersized stilts can cause the
ware to shift in the kiln and could cause the piece to deform.
Dry-footing should be discouraged at all times (excluding
stoneware and porcelain, where it is required.) Dry-footing is
the removal of the glaze on the bottom surface where the ware
touches the kiln shelf. While some pieces cannot be stilted, dry-
footing is a last resort procedure.
Glaze is like sugar that is being made into candy. Think of
the sugar starting out as fine granules. As heat is applied to the
sugar it starts to melt, bubbling and boiling, as glaze does on the
ware. This is why the heat work is so important. The glaze
needs to come to a boil and then smooth out to a hard
crystalline surface, just like candy. If the firing procedure is
interrupted at any point, problems can occur, such as pinholes,
bubbles trapped in the glaze or blisters in the surface. If the
cooling is too rapid, these problems do not have a chance to
heal themselves.
Mayco glazes are referred to as oxidation glazes; they
require oxygen to develop fully in the kiln environment. The
absence of oxygen can change the color greatly, especially for
bright reds. Give glazed pieces plenty of room to breathe
during the firing. Giving reds a bit more room than normal aids
their development. If glazed pieces are put too close to one
another the oxides in the glaze react with the adjacent color; this
is referred to as fuming, flashing, blushing or shadowing. It
may also appear as a yellow cast on light colored glazes.
Unlike some competitor’s glazes, all Mayco glazes can
be fired with one another, including green and reds. Mayco's
bisque glazes, bright reds, and oranges due not require a
special firing when mixed with other glaze colors like greens.
However these pieces should be put on higher stilts to allow
for better air circulation. Give them wider berths in their kiln
placement, including the placement of shelves over those
pieces. The use of other companies’ colors used in
conjunction with Mayco colors can not be assumed or
ensured; test fire their compatibility prior to applying colors
on a project.
Whether firing whiteware (earthenware), porcelain,
stoneware or glazes, make certain you have a thorough
knowledge of what you are placing in your kiln. The proper
precautions and actions will ensure a product you will be
proud of. Now you are all fired up to operate your own kiln.
Tips for Firing and
Surviving in the Classroom
Bisque Firing
Dry greenware can touch in a bisque kiln. Bowls and cups
are best stacked rim-to-rim or foot-to-foot. Stack smaller pieces
in larger pieces. Don’t stack objects that are a tight fit together
or have projects touch the elements.
12
Firing