Spectrum Pottery Glazes Usage and Tips
Unfortunately there is no perfect set up for dipping glazes that
will work in all applications. The goal is to find the right set
up, in terms of viscosity and density (or specific gravity) that
will provide the finish you want to achieve. Ideally for a clear
dipping glaze you are trying to deposit the least amount of glaze
that will provide a clear glossy finish over the entire piece. An
opaque glaze will require a thicker coating of glaze to provide
consistent color and finish over the whole piece. There are several
factors that affect the thickness of glaze deposited on a piece.
The first is the bisque temperature of the piece. The hotter a piece
is bisqued, the tighter the body becomes and the less glaze is picked
up by the piece. The formulation of the clay body being used can
also affect the tightness or porosity of the piece and therefore
the amount of glaze deposited. The other significant factor is dryness
or wetness of a piece. The drier a piece is when it is dipped the
more glaze it will be able to pick up. This is particularly important
for pieces with a lot of underglaze decoration on them. If the underglaze
has not been allowed to thoroughly dry before dipping the decorated
areas will not pick up as much clear glaze as the areas of bare
bisque and may look rough or partially unglazed after firing.
Due to the great variety of conditions under which dipping glazes
can be used our recommendations for dipping glaze set up should
be used only as guidelines. They are generally appropriate in the
majority of cases. However, to obtain the best results for your
particular conditions, the first time you use a new dipping glaze,
you should dip a series of test pieces in order to determine the
range of density and viscosity readings that provide the best results.
The simplest way to do this is to start with a set up slightly thicker
than recommended and add water in small increments, such as 2 oz.
per gallon of glaze being adjusted. Dip a test piece and record
density and viscosity after each addition of water. Be careful not
to add too much water as glazes that are too thin can settle in
the bottom of the container. The results of the fired test pieces
should show you the range of density and viscosity readings that
produce good results.
Specific gravity is a measure of the density of a liquid measured
in grams per cubic centimeter. Water has SG=1.0, which means that
1 cc of water weighs 1 gram. To calculate the SG of a glaze divide
the weight of the glaze by the weight of an equal volume of water
(i.e. weigh 100 cc of glaze and divide by 100 to obtain the SG of
the glaze). This can be done easily using a scale such as an Ohaus
triple beam balance and a plastic measuring cylinder cut off at
the 100 cc mark. Fill the cylinder to the top, weigh it, remember
to subtract off the weight of the cylinder, and divide by 100 to
obtain the SG.For a dipping glaze a combination of viscosity and
SG is required that will result in the correct thickness of glaze
being deposited on the piece in one dip application.
The usual range of SG for a dipping glaze is 1.50 to 1.70, the
lower end of the range being for clear glazes and the higher end
for opaque glazes. Various other factors will affect the determination
of the appropriate SG of the glaze. These factors include whether
the piece to be dipped is greenware or bisque, the bisque temperature,
whether the piece is made from pug clay or casting slip, the moisture
content of the piece, and anything else that affects the ability
of the piece to absorb glaze. By comparison brushing glazes usually
have SG in the range of 1.45 to 1.60.
Glaze viscosity (or thickness) can be measured in terms of the
number of seconds that a given volume of glaze takes to flow through
a hole of a certain diameter. The easiest way to measure viscosity
is by obtaining a viscosity cup, also called a Zahn cup, from a
hardware or paint store plus a stopwatch. The viscosity cup is a
small container with an open top and a bowl shaped bottom with a
small hole to allow the glaze to flow out. The cup has a handle
so that it can be dipped in the glaze. Fill the cup to the brim
while holding a finger over the hole. Start your stopwatch when
you remove your finger from the hole. Record the time it takes until
the stream of glaze breaks at the bottom of the cup and only drips
remain. The number of seconds is a reliable measure of the viscosity
of the glaze. Dipping glazes generally have viscosities in the 40
to 60 second range, although many of the factors mentioned previously
with respect to SG may also call for viscosities outside of this
range.
By maintaining the specific gravity and viscosity of a glaze within
a predetermined range it is possible to control the thickness of
glaze that is deposited on the piece. This is especially important
with respect to dipping glazes versus brushing glazes since you
want to achieve the desired result with a single dip. Also dipping
glazes tend to thicken up as they are used because water is sucked
out of the glaze into the bisque as pieces are dipped. Therefore
it is necessary to monitor viscosity and SG on an on-going basis,
not only in the initial set up of the glaze.
The control of glaze thickness is essential for the production
of consistent, high quality results. Glazes that are applied too
heavily or too lightly are the cause of any number of glaze defects
including pitting, crawling, color variation and rough spots. The
most reliable way to check glaze thickness is with a thickness gauge
which can be purchased through an instrument supply catalogue. Hold
the gauge on the surface of the glaze. When the button is pressed
a needle penetrates the glaze to the bisque surface and displays
a reading of coating thickness in either millimeters or thousandths
of an inch. However for many potters and ceramists the simple scratch
test is sufficient. After dipping a test piece and letting it dry,
scratch through the glaze down to the bisque with the tip of a knife.
The correct thickness of glaze is usually about the thickness of
a thumbnail.
Testing pieces to determine the range of glaze thickness that will
produce the desired results can save many hours of unnecessary troubleshooting.
Once the appropriate thickness range has been determined it can
be correlated to a range of viscosity and SG values that produce
the correct thickness. Maintaining viscosity and SG within the desired
range can usually be achieved by the addition of small quantities
of water. There are other materials, such as Spectrum's Glaze Suspender(1071)
for thickening and Glaze Thinner(1072), that can be used rather
than water.Achieving the proper glaze thickness on a consistent
basis through the control of viscosity and specific gravity is one
of the keys to avoiding many common glaze defects.
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