THE ART OF SLIPMAKING
printed with the
permission of laguna clay company
Making good slip is the first and most vital step in a successful
slip casting operation. Although there are those who will say they
have been using the same dixie cup for the last ten years to
measure sodium silicate and a nail on a wooden barrel for a water
marker, this approach leaves far too much room for error. Accurate
measurements of weight and specific gravity are important. It
doesn't take a ceramic engineer to make a good quality casting
slip, but slip mixing is a chemical process which is affected by
even the slightest changes in temperature, humidity and the
chlorine and chemicals which are added to water supplies. Changes
in conditions such as weather and water can sometimes affect the
mixing formula but if you adhere to the following procedures, you
will consistently make good slip. For best results, make your slip
24 hours before using it for casting.
EQUIPMENT NECESSARY FOR SLIPMAKING
An efficient casting room should be equipped with the following
- A slip mixer. With a motor and blades which are capable
of properly mixing your batch of slip.
- A gram scale. A postage scale is not accurate enough.
- A viscometer. Developed to aid in controlling the
quality of slip. It is used to weigh the slip, which will show the
ratio of clay to water in the slip, and to measure the viscosity of
slip, which indicates the amount of slip that flows per second.
- A liquid ounce. Sodium silicate for instance, must be
measured by the liquid ounce, not by weighing it on a scale. Four
ounces of sodium.
- Measuring cup. silicate, measured by volume, weighs
approximately five ounces on a scale.
- A hydrometer. The hydrometer, an inexpensive and
exceptionally easy to read instrument which displays the viscosity of
the slip as it relates to specific gravity.
- Respirator mask(OSHA approved)
- A minimum of 60 mesh sieve A talisman sieve is an
invaluable aid in
screening your slip. You simply turn the handle and the slip is
forced through the screen.
CHEMICALS NECESSARY FOR SLIPMAKING
- SODIUM SILICATE Also known as electrolyte, is used for
deflocculation. (Always dilute the amount of sodium silicate
suggested with a like amount of water to yield a fifty-fifty
- SODA ASH For deflocculation. (It works with sodium
- BARIUM CARBONATE To neutralize sulphates present in clay
and water. (German barium recommended)
(Toxic in raw form)
- DISPERSAL May be necessary for additional
ALWAYS MEASURE DRY INGREDIENTS BY WEIGHT AND MEASURE SODIUM
SILICATE AND DARVAN IN LIQUID OUNCES.
- SODIUM SILICATE
Sodium silicate is a chemical used to
deflocculate casting slip. NEVER ADD UNDILUTED SODIUM SILICATE TO
THE SLIP BATCH. Too much sodium silicate causes over-
deflocculation. With the batch formulas we have given the ranges
from the least to the greatest amounts of sodium silicate nedded
to deflocculate each batch. If you have had no experience with
making slip at your location, measure the least amount of sodium
silicate needed with your first one or two batches and "add as
needed". It is easier to do rhis than try to adjust a batch to
which too much sodium silicate has been added. Keep records of the
amounts you mix and mark that figure on the "Sodium Silicate Range"
which we list with the formulas. This is important for future
NEVER ADD PREMEASURED SODIUM SILICATE TO THE BATCH ALL AT ONE
TIME. ADD ONLY IN SMALL AMOUNTS. Sodium silicate may be stored
for extended periods in tightly closed containers. (Aluminum,
galvanized iron or zinc containers SHOULD NOT be used.)
- SODA ASH Soda ash works with sodium silicate to aid in
deflocculating. The correct combination of the two will give proper
casting qualities. Soda ash works to dissolve lignite in clay. It
works as a deflocculant, but if only soda ash is used as a
deflocculant, your clay will become sticky. If soca ash is not
stored in tightly closed containers, a chemical change occurs and
it becomes sodium bicarbonate (baking soda). If this occurs it will
then act as a flocculant (it will thicken) instead of as a
- BARIUM CARBONATE It is usually necessary to use barium
carbonate in your slip batches to neutralize sulphates that may be
present in the clay or in the water and sulphates leeched from
molds used in casting. Because of natural changes which occur in
the mining of the raw clays, one should add barium
- DISPERSAL Dispersal is an organic deflocculant which can
be used with less fear of over-deflocculation. It is especially
effective in hard water areas.
Formula using 100 lbs. clay blend (Yield: 10 gallons)(Start
with MINIMUM AMOUNTS shown in formula)
Clay Blend100 lbs.
Barium Carbonate1/2 ounce (14.75 gr.)
Soda Ash1 ounce (28.35 grams)
Water5 to 5 1/2 gallons (19.5 to 21.45
Sodium Silicate2 1/2 to 3 1/2 fluid ounces (You
must dilute this amount with a like amount of water.
This will yield 5 to 7 ounces diluted sodium
*SODIUM SILICATE SHOULD ALWAYS BE DILUTED 50/50 WITH WATER
BEFORE BEING ADDED TO THE BATCH.
The hardness or softness of the water in your community will affect
the results of your casting slip. Therefore, we will avoid
recommending precise amounts of the ingredients in our formulas.
Instead, a range of quantities will be given. We recommend that you
begin with the minimum amounts of each ingredient and if
adjustments are needed, slowly and carefully add additional
- Before you begin mixing, put on a good quality respirator mask
to protect you from inhaling dust.
- Measure water carefully and pour water indicated for you size
batch into the slip tank.
- Accurately measure the minimum amount of sodium silicate
indicated for your batch and add a like amount of water (i.e. 3 oz.
sodium silicate +3 oz. water) in a measuring cup. SET ASIDE.
NEVER ADD SODIUM SILICATE FULL STRENGTH TO A BATCH OF SLIP.
- Accurately measure the amounts of soda ash and barium
carbonate indicated for your size batch. SET ASIDE.
- Add barium carbonate and soda ash (which you have previously
measured in step 4) into one quart of warm water. (Warm water aids
in mixing the chemicals.)
- Turn mixer on. Add diluted soda ash and barium carbonate
(which you prepared in step 5) to the water which is in the tank.
Mix for 5 minutes.
- While the tank is still mixing, slowly add 1/3 of the clay
indicated for your size batch to the water.
- Add 1/3 of the silicate solution (as mixed in step 3) to the
- After mixing a short time, allowing the sodium silicate to
thin the batch, add 1/3 more of the clay. Allow clay to mix into
- Add 1/3 more of the sodium silicate solution. This will again
thin the batch.
- After mixing a short time (approximately 10 minutes), add the
final 1/3 of your clay to the mixer.
- The last 1/3 of the sodium silicate should be added at this
- The mixing time should be approximately as follows:
100 lbs. - 2 hours 1000 lbs. - 4
300 lbs. - 3 hours 2000 lbs. - 5
Mixing will cause the sodium silicate to blend into the clay and
water. Therefore, the batch will thin as it is mixing. For this
reason, the slip maker should not judge the consistency of the slip
until it is completely mixed or the batch could becone over-
deflocculated and too thin. After the batch is completely mixed,
test it as recommended in the testing section. EYE-BALLING slip is
an extremely inadequate method of confirming quality.
Proceed with testing. However, we do not recommend that any
adjustments be made to the formula until 24 hours after the batch
is made and tests are repeated.
TEST 1 HYDROMETER
The hydrometer is an inexpensive and exceptionally easy to read
instrument used for measuring the viscosity of a deflocculated slip
whose specific gravity has been predetermined by a weight test
prior to using the hydrometer. After turning off the mixer, simply
place the hydrometer in the batch of slip. It will sink and stop
at teh reading indicator. It will then float like a bouy. A normal
reading is between 1.76-1.82. The thicker the slip, the greater the
reading and the less the instrument will sink into the batch. The
thinner the slip, the deeper the hydrometer will settle. For
example, water has a specific gravity of 1.00 and a hydrometer will
nearly submerge. The opposite occurs in slip. Remember, liquid slip
is not really a liquid. It is a mixture of clay, suspended, not
dissolved, in proportions. Specific gravity is the density of the
slip. Remember that the reading on the hydrometer will reflect the
thickness and thinness of the slip which is effected by the amount
of deflocculant used during the mixing. The reading on the
hydrometer should reflect the weighed specific gravity. For
instance, if a pint of slip weighs 29 ounces it will have a
specific gravity of 1.75. If a pint of slip weighs 30 ounces, it
will have a specific gravity of 1.80. The hydrometer should have
a similar reading. If the hydrometer reading is above 1.80 it
indicates that your slip is not properly deflocculated. In reality,
the specific gravity, as determined by the weight test, may be
correct and should not be altered with further additions of water
HOW TO USE THE HYDROMETER
Be sure the hydrometer is completely clean and dry. There should
be no trace of water on its outside surface. Hold the hydrometer
gently between your index finger and thumb. Keep the hydrometer
pointed straight down and not tilted. Slowly lower it into the
slip. As the bulb is lowered into the tank and submerges into the
slip, release the hydrometer. (Take care not to push the hydrometer
to an angle as you let go). Let the instrument settle to its
natural level and read the indicator. rinse the hydrometer clean
after each usage. Allow it to dry before re-using.
TEST II WEIGHING TEST
If you have followed the procedure we recommended for mixing, the
consistency of your slip should be either correct or slightly under
deflocculated. To find out, fill a pint jar with slip and weigh it
on the scale. (be sure that you first weigh the jar itself, so you
can subtract its weight from your measurement.) The correct weight
for a pint of slip is between 28.9 oz. (820 grams) and 29.2 oz.
(828 grams). This simple measurement, along with the hydrometer
reading, quickly indicates whether the batch has enough clay and
water and whether it has been correctly deflocculated. A pint of
slip that weighs 29 oz. will have a specific gravity of 1.75. A
pint of slip that weighs 30 oz. will have a specific gravity of
1.80. The hydrometer reading should correspond to these
TEST III VISCOMETER
If you are using the viscometer, you may eliminate the above
weighing test and proceed with the following instructions. The
viscometer was developed to prevent problems related to slip
casting. The viscometer test will determine teh ratio, by weight,
of clay to water, and the viscosity (the rate slip flows per
USING THE VISCOMETER FOR THE WEIGHING TEST
- STEP I Fill the flask with slip to
the 500 ml mark.
- STEP II Weigh the slip and the flask.
- STEP III Subtract the weight of the
flask from the total weight (glass flask weighs approx. 195 grams;
plastic flask weighs 89 grams). The result will be the net weight of
500 ml. of slip. Normal slip weight will be 860 to 900 grams.
Experience will teach you which weight is most appropriate for your
casting conditions. If weight is under 860, more clay blend is needed.
If it is over 900, more water is needed. Divide the gram weight of the
slip by 500 to establish the weighed specific gravity. For example: If
your slip weighs 875 gramö500 the specific gravity will be 1.75.
When your slip is properly deflocculated the hydrometer will read
approximately the same.
USING THE VISCOMETER FOR THE VISCOSITY TEST
- STEP I
Replace the stopper into the already filled
- STEP II
Hold your finger over one hole in the stopper
and turn the viscometer upside down.
- STEP III
Release your finger from the hole and time the
flow of slip until all the slip has drained from the flask to the
fifth drop. Normally the slip will flow through in 100 to 130
seconds per 500 ml. A fast flow time (thin slip) will probably lead
to a slower setting time, whereas a slow flow time (thick slip)
will set up faster in your molds.
It is recommended that slip be screened through a 60 mesh sieve
REPEAT TESTING AFTER 24 HOURS. MIX BATCH FOR 30 MINUTES THEN
- A. If the slip weight is normal, it is highly unusual for the
flow rate to be less dthan 100 seconds per 500 ml.
- B. If the slip weight is normal and the flow time is more than
130 seconds, you may need a little more sodium silicate or
- C. After testing, always wash your viscometer with cold water
and store at room temperature.
Sodium silicate, while it is the most commonly used deflocculant,
has a very narrow range of deflocculation. Therefore, one can
easily over-deflocculate slip. The addition of sodium silicate to
a slip batch thins (deflocculates) it. When a certain point is
reached, the addition of more sodium silicate will begin to
rethicken (flocculate) the slip. If your slip has not reached the
over-deflocculated state, the addition of a few drops of a sodium
silicate water solution will thin the batch, but if you have gone
beyond the allowable linits of sodium silicate water solution and
your slip has become thick because of it, the addition of more
sodium silicate will actually thicken the batch more. If your slip
is badly over-deflocculated, it is most prudent to discard it as
it is very difficult to correct over-deflocculation.
Dispersing agents such as "darvan", commonly used for
deflocculating, can be added after you have added the maximum
amounts of sodium silicate. You may add as mush as 1 ounce of
dispersal per 100 lbs of dry clay without fear of over-
deflocculation. DO NOT ALLOW DISPERSAL TO FREEZE.
ADJUSTING THE BATCH
If extra clay is added, mix the batch for the full recommended time
(i.e., 3 hours for a 300 lb. batch). On the other hand, your slip
need only be mixed for fifteen minutes if only extra water or
sodium silicate is added. If your weight and viscosity readings are
slightly higher or lower than the ranges given in our testing
procedures, casting under your conditions will be the ultimate
test. IF YOUR SLIP CASTS WELL YOU SHOULD NOT MAKE ANY ADJUSTMENTS.
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