Dawnmist Studio is proud to introduce a unique range of Crystalline
Glazes, which bring spectacular large-crystal effects within the reach
of almost any potter, without the difficulties traditionally associated
with this type of glaze. Any kiln capable of firing to Cone 8
in oxidation and equipped with a basic pyrometer can be
used; small, fast-firing electric kilns are ideal.
To get the best performance from these glazes, please read these
instructions thoroughly. Even the best-designed crystal glazes are more
temperamental than ordinary stoneware glazes, and this booklet is
the result of considerable experience in using them.
- These glazes are supplied in powdered form and are mixed with
water in the usual way (manually or by electric mixer) followed by
sieving. 60 or 80 mesh is recommended. Any small amount of
material remaining in the sieve after thorough brushing to propel the
glaze through the mesh may be discarded. The need for thorough mixing cannot
be emphasized enough: proper crystal formation depends on having all
glaze ingredients thoroughly mixed and evenly suspended. An electric
mixer is strongly recommended.
- It will usually be found that between 1.0-1.3 litres of water is
required per kilo of dry glaze, depending upon the individual glaze and
the desired consistency of slop glaze preferred --- which, of course,
depends on your preferred method of application. The normal glaze consistency
is best described as "thicker than milk, thinner than cream".
- These glazes are suitable for application by brushing, pouring, dipping
or spraying. Crystalline glazes should be applied thickly. Glaze adhesion can
be improved if necessary by adding acrylic medium (see Troubleshooting
section for details) and the glazes can be thickened for easy brushing
by adding 0.5-1% of water-soluble PVA adhesive, i.e. 5-10 grams per
kilo of dry glaze weight.
- Crystalline glazes are very low in clay content and tend to settle
out quickly. To reduce this problem, you will find a sachet of
flocculant (white crystals, labelled F)
enclosed within each bag of powder glaze. This is a measured
quantity (0.5%) and is adequate to flocculate the entire quantity of
powder glaze when made up into slop form. Dissolve the flocculant in a
little hot water and add to the slop glaze before sieving. Flocculation
aids glaze suspension, making the glaze creamier and more pleasant to
work with. We also recommend frequent stirring of slop glazes when in
use, to ensure that the heavier ingredients stay in suspension.
- Each pack of crystal glaze also includes a sachet of Crystallisation
Promoter (fine white powder, labelled P). This ingredient allows you to control
the intensity of the glaze's tendency to form crystals, to fine-tune the results of
glazing. The Promoter is mixed with the powdered glaze prior to adding water, or may
alternatively be mixed with water and added retrospectively to increase the glaze's
tendency to crystallise. Because the Promoter is supplied separately from the glaze,
you have the flexibility to effectively fine-tune the formulation of the glaze to suit
your clay body and to obtain different effects.
Experience at Dawnmist Studio has found that it is generally desirable to add most or
all of the Promoter when working with translucent porcelains, otherwise insufficient
crystallisation is likely. Conversely, when using semi-porcelains or stoneware, we have
found that too much Promoter can lead to solid masses of interlocking crystals ---
an effect that may or may not be desired, according to your artistic intentions. If
this effect is not desired, but isolated large crystals are preferred, the glaze may
function best with the Promoter added sparingly, or even totally omitted. The Promoter
supplied is a weighed quantity, 2% of the dry weight of the glaze, and may be
subdivided if required. Please ensure that the Promoter is thoroughly mixed with the
rest of the glaze.
A little experimentation is the best method to explore the possibilities of these glazes
on your particular clay body, if you have not used the range before, as well as to
establish how much Promoter best suits your needs. It is simple to mix up a sample of
glaze without any Promoter, then add Promoter stepwise, testing the glaze at each
addition of Promoter until you find the addition that gives the type of crystals that
you prefer. If you start by making up five small test tiles, ranging from no promoter,
through one-quarter, one-half, three-quarters, and finally the full amount of Promoter,
you will get a clear idea of how the amount of Promoter used affects the result with
your particular clay body and firing process. If you wish to make up only a small
amount of glaze for this kind of initial test, simply remember that the 'full dose' of
Promoter is 2% of the (dry) weight of glaze and add it pro rata: for example,
if you wish to make up a test batch of 50 grams (dry) of glaze, weigh out exactly one
gram (2% of 50g) of Promoter, divide it into four equal 'doses', then add each dose in
turn to the slop glaze (mixing in well), at each step glazing a small test tile with the
mix. Once you have established how much Promoter is best for your application, you can
readily mix up large batches of glaze with the same percentage of Promoter mixed-in each
time, with no need to repeat any tests.
- These glazes are intermixable to produce different colours: for
example, a combination of NW903 and NW904 produces shades of turquoise or
cyan, while NW901 and a small addition of NW904 produces pale blue crystals.
An entirely different palette of colours can be obtained by reduction firing; this process
is described in its own section below.
2 Glaze Application
- Before and during glazing, make certain that the glaze is
well-mixed. Sedimentation of the heavier ingredients can upset the
precise chemical balance required for good crystallisation. A
side-effect of using less (or none) of the Crystallisation Promoter is
that the glaze will tend to settle more rapidly and will need frequent stirring.
- The crystalline glazes perform best over a very smooth and preferably white clay
body, and are especially suited to porcelain and semi-porcelain. Semi-porcelain (e.g.
Potterycrafts P1230) gives a surface quality directly comparable to porcelain, but is
much more workable than true porcelain, and is highly recommended. A very smooth,
ungrogged white stoneware (such as Earthstone Extra Smooth) is also suitable,
particularly if the bisque surface is sanded smooth before glaze application. This will
tend to give a mass of small to medium crystals instead of a few large isolated ones,
though this effect can be adjusted by omitting some or all of the Crystallisation
- A different effect can be obtained if the glazes are used over a
rougher surface, e.g. a grogged clay. In this case, the surface
roughnesses act as nucleation sites for crystal formation, and the
result is a tight mass of needle-like crystals.
- All crystalline glazes are by nature very fluid when molten, and it is inevitable
that some glaze will run off the bottom of the pot. As well as leaving the bases
unglazed, we recommend the use of appropriate stilts or supports and the use of 'catch
bowls'. A 'catch bowl' is typically a crudely-made shallow bowl a little larger than
the diameter of the pot, above which the pot is stood (supported by a suitable stilt
placed in the catch-bowl), and which captures any molten glaze that runs off the pot.
Low-cost stoneware clay is ideal for catch bowls, and even pinch-built bowls are quite
adequate. Catch bowls and stilts can generally be re-used many times, though if the pot
being glazed is particularly tall and unstable it might be preferable to make a one-use
stilt/catchbowl that is bisqued to the same temperature as the pot and therefore shrinks
exactly in step with the pot, eliminating any small differential movement that may cause
the pot to topple as it shrinks.
To make such a one-use catchbowl/stilt combination, first make a shallow bowl a little
larger than the foot of your pot, preferably out of the same clay body that has been
used for the pot (although very often a cheaper clay may be sufficient). Insert pins
made of high-temperature 'stilt wire' (Nickel-Chrome (Nicrome) or Kanthal element wire
works well) into the soft clay so that they support the base of the pot --- and make
sure that the base of the catch-bowl is thick enough to support the weight that your pot
will place on the pins. Then bisque-fire the bowl along with the pot, and stand the pot
on the bowl's pins for the crystal firing. The bowl should shrink exactly in step with
the pot, avoiding any risk of toppling.
A similar method can be used to make re-usable stilts and catch-bowls; the stilt and
catch-bowl are best made separately, both for flexibility in firing different shape/size
pots and for ease of renewal if, say, a catch-bowl breaks in firing --- if they are
separate, you do not need to renew the stilt as well. In this case, a catch-bowl can
be made thin all over to save on clay, while the stilt consists of a thick slab of clay
with wire pins pushed in as for the single-use method above. We have found that if a
good strong grogged stoneware is used for the stilts and catch-bowls, they can normally
be re-used many times
There are many other techniques known for stilting crystal-glazed pots, and examples
will be found in many pottery books. The methods described here are the ones that we
have found best after extensive experimentation, and are the methods used for Dawnmist
Studio's own pots. We normally use re-usable stilts and catch-bowls, and find that
only exceptionally top-heavy pots ever require the single-use type. We use 2mm diameter
Nichrome wire for stilt pins, usually 25mm in length and pushed 10-12mm deep into
the stilt base. After firing, very slight flexing of the pins will easily crack off any
blobs of glaze and permit the pot to be removed.
- Any blobs of glaze left on the bottoms of pots can be ground down with a fine
carborundum stone (preferably power-driven for speed, though manual grinding is
certainly possible) after the stilts are removed. Be careful not to create too much
heat while grinding, as this may crack the glaze or even the pot; if in doubt, it is
best to use a water-cooled grinding wheel, as these are capable of removing surplus
glaze very rapidly with minimal risk of harm to the pot. In tests at Dawnmist Studio,
excellent results have been obtained by using a fine carborundum stone at 3000 rpm with
trickle-fed water-cooling; alternatively, we have found that an angle-grinder fitted
with a fine stone-grinding disc works well, and with reasonable care can even be used
without the need for water cooling.
- Because of the fluidity of the molten glaze, it may be worth coating the pots in
such a way that the glaze is thicker at the top than at the bottom, so that it evens
itself out during firing. Any 'tide marks' resulting from different numbers of coats
will disappear during firing. Crystalline glazes should be applied quite thickly to
ensure good crystallisation. Excessively thick application can cause problems, however
(see Troubleshooting section below), and it is difficult to give specific advice
on number of dips or coats required, as bisque ware varies considerably in its porosity.
As a general rule, crystalline glazes should be applied rather more generously than
normal stoneware glazes.
- Although these glazes will form crystals spontaneously with the
correct fire-down, it is also possible to 'seed' crystal formation with
small dabs of Dawnmist Studio Seeding Agent (NW900) applied over the glaze film.
To do this, mix the Seeding Agent with water to form a paste, and apply
small blobs of this as required, using a small paintbrush.
Crystals will tend to grow preferentially centred upon where the Seeding
Agent is dotted, though spontaneous crystallisation will still occur. A
faster fire-down and smoother bisque ware will reduce the amount of spontaneous crystal
formation, as will using less of the Crystallisation Promoter.
- Additionally, the smoothness of the bisque surface affects the
crystallisation: a sanded-smooth (semi-)porcelain will be more likely to
form individual large crystals, while an un-sanded bisque pot will be more
likely to produce many, smaller crystals. Even with porcelain bodies,
sanding the bisque to perfect smoothness improves results appreciably
where individual large crystals are desired. Be sure to remove surface dust after
sanding, otherwise this will be absorbed by the glaze and will impair
crystal formation. Be sure to take appropriate anti-dust measures when sanding
bisque ware --- preferably, do it wet, since bisque dust is hazardous if inhaled.
This section lists problems that may be encountered, along with a list
of likely causes and appropriate remedies.
Small, rough crystals and/or matt surface finish
- Glaze not mixed and sieved thoroughly, or glaze has settled, so that
glaze composition is incorrect.
- Glaze applied too thinly.
- Starting (upper) temperature of the fire-down was too low.
- Proper maturation temperature not reached (underfiring). Maturing temperature
can easily be checked by using
Orton cones --- Cone 8 should bend fully over; an
overfire of up to one cone will not be likely to cause problems.
Remember that pyrometers are not always accurate.
- Glaze may have absorbed alumina from loose clay dust on the
bisque, or from the clay body itself. Try sanding the bisque smooth and
removing all dust; also try other clay bodies --- some are more suitable
No crystallisation occurs (glossy glaze)
- Starting temperature of fire-down was very much too low.
- Fire-down has been performed much too fast.
- Glaze has settled or is not properly mixed.
- Insufficient addition of Crystallisation Promoter for your clay body.
- Glaze may have absorbed alumina (see above).
- Excessive outgassing from clay body; try a higher temperature
and/or slower bisque firing, or a soak at maturation temperature.
Glaze cracks or peels as it dries
- Excessively thick glaze application.
- Excessively fast, forced drying of the glaze.
- Poor adhesion.
Cracking/peeling of the unfired glaze film is a risk with all crystalline glazes, as
they are very low in clay content compared to conventional glazes, and consequently do
not always adhere well. They also exhibit greater drying shrinkage than most glazes
because of their unusual, low-clay composition. A number of approaches can be used
besides the obvious one of making the glaze coating thinner (but note that making the
coating too thin will lead to other problems in the fired glaze!):
- Even quite large cracks are not a problem: since crystalline glazes are extremely
fluid, cracks will 'heal' during firing, and crystalline glazes very seldom exhibit
'crawling'. Even a glaze film that "looks like a jigsaw puzzle" will usually fire
perfectly evenly because of this fluidity.
- Very large cracks can be smoothed over with a damp finger or sponge.
- Apply thin coats, allowing each to dry thoroughly before applying
the next. This can reduce cracking.
- Dampen the bisque slightly before applying glaze, to slow the drying process
and reduce absorption of water by the ware.
- Add Acrylic Medium to the glaze as a plasticiser and adhesive. This material,
typically used for preparing acrylic colours for fabric-painting, is obtainable from
most art shops. A typical addition of 2-5% by dry weight of glaze will improve the
adhesion greatly. Acrylic Medium tends to thicken the glaze, and it may be desirable to
add more water to avoid excessively thick coats. PVA is not suitable for this
application unless the glaze is to be brushed-on, since it thickens the glaze
excessively. Acrylic Medium is the recommended solution to adhesion problems and is
used routinely at Dawnmist Studio. Other glaze binders, such as vegetable gums (of
which 'gum arabic' is probably commonest), can also be used in the same manner --- but
be aware, if slop glaze is to be kept for an extended period, that vegetable gums are
bio-degradable unless a preservative is added.
5 Reduction Firing
Although these glazes are designed to fire in oxidising conditions (as
found in an electric kiln), a variety of new and stunning colour effects
can be obtained by applying a reduction atmosphere during the cooling
process. This can be performed by firing in a gas-fired kiln, by
modifying an electric kiln to perform reduction, or by a low-temperature
post-firing in a gas (e.g. Raku) kiln after electric firing. Whichever
method is used, it is important to complete the firing to maturity, and
the controlled cooling over the crystal growth range, in oxidising
conditions and only to switch to reduction after crystal growth is
complete: crystals will not grow well in a reduction atmosphere.
Reduction is entered at about 800°C and ceased at about 650°C (firing-down
in reduction, in other words).
The most obvious of the three techniques is to use a gas kiln for the
firing, so that reduction can be obtained during cooling. This does
work, so long as an oxidising atmosphere is rigorously maintained during
maturation and crystal growth, though given this constraint it is often simpler to
obtain good results with electric firing. Gas kilns can be a little
temperamental even in the hands of the most experienced potter!
If a gas kiln, even a Raku type, is available, it is possible to perform
the reduction in a separate firing. In this case, the main (electric)
firing is performed in the usual way to grow the crystals, and a
separate gas firing is then performed to take the ware up to 800°C and
cool gradually to 650°C while maintaining reduction.
There has been much interest recently in performing reduction firing in
an electric kiln, and many articles have appeared in print and on the
Internet regarding methods for this. We have used this method at
Dawnmist Studio, using a kiln whose elements and structure have been
reinforced against reducing atmospheres by means of a special coating (ITC100 and
ITC213) and to which a small LPG burner has been attached to create
reduction conditions. In this case, the kiln is fired in pure electric
(oxidation) mode until it has cooled down to 800°C, at which point the
gas burner is lit and the kiln fired-down through the reduction range.
The electric elements can be used in addition to the gas burner to
regulate the speed of the fire-down and give the ware a sufficient
exposure to reducing conditions.
Whichever method is used to obtain reduction, careful control of
atmosphere is required during reduction. If reduction is too heavy, soot
deposits may result due to the low temperature reduction, and these will
have to be removed. Different levels (and durations) of reduction produce a wide range
of colour effects, and an oxygen probe is a valuable addition to the
kiln to make this repeatable (details of a low-cost oxygen probe can be
obtained from Dawnmist Studio on request).
The colours most suitable for reduction firing are NW903 Mint Green,
NW908 Turquoise and NW910 Pastel Green. These can produce a variety of
reddish shades in heavy reduction (NW903 purplish, NW908 maroon, and
NW910 pinkish-red); additionally, NW908 in light reduction can produce a
subtle and attractive olive colour. In addition, several other colours
have the potential to change colour in reduction, particularly NW902
Brown Sugar, NW906 Midnight and NW909 Silver-Grey. There is considerable scope for
experimentation with reduction firing of crystal glazes, a field which
has so far received very little attention.
6 A Final Word...
- Glazes in the NW900-999 series are lead-free, and if used and fired correctly
can meet UK standards for use on food vessels. Responsibility for testing
rests with the potter using the glaze, as conditions of use are beyond our control.
- Appropriate health and safety measures should be followed,
including wearing a mask while handling dry powder glaze. A
Health and Safety sheet below details the cautionary notices applicable to
- Recommended further reading for those interested in the art of
crystal glazing: "Crystalline Glazes", Diane Creber, AC Black, London, 1997.
- We want you to get the best out of our glazes. In case of any
difficulties or queries which are not covered by these instructions,
please contact Dawnmist Studio and we will be pleased to help.