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 (approx. 1260°C) 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.

1   Preparation

• 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.

• 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 Promoter.
  
  
• 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.

• Crystal glazes require a special firing sequence: fast-fired to maturation temperature, an optional soak, then cooled rapidly to the top of the 'crystallisation band' of temperature, then 'fired-down' (controlled cooling) through the crystallisation band, after which the kiln is switched off. Some kiln controllers are sophisticated enough to program such a firing directly; with simpler controllers the firing can usually be divided between two programs (a regular fast firing to maturation, then a downward ramp through the crystallisation range --- many controllers are quite happy to perform a downward ramp where a normal firing would specify an upward ramp). With a kiln-sitter or other type of cut-off firing control, the 'fire-down' is obtained manually by switching the kiln back on and adjusting the power regulator to produce the necessary controlled fall in temperature, by observing the pyrometer.
  
  
• There is scope for experimentation with different crystallisation bands and different speeds of fire-down (a longer fire-down produces larger crystals), and it should also be remembered that pyrometers do vary slightly in their readings, and different glazes in the range may behave slightly differently. The following table lists recommended basic firing regimes which can be used as a starting point and will give good results with most kilns:
  
  

  Glaze:	Crystals:	Mature at:	Fire-down °C:	At °C/hr
  NW901-10	Small-medium, unseeded	Cone 8	1130-1030	30-40
  NW901-10	Large, unseeded	Cone 8	1130-1030	20-25
  NW901-10	Large, seeded	Cone 8	1130-1030	25-30

  NW902/3/4/8/10 will be found to crystallise a little slower than NW901/5/7/9 due to the colourants used. NW906 is significantly slower than all the others, since a slightly different formulation is used in order to obtain the 'midnight' colour scheme.
  
  

• A fast fire-down tends to produce smaller, individual crystals floating on a glossy background, while a slow fire-down tends to produce a surface completely covered with large interlocking crystals. The amount of Crystallisation Promoter used also affects the results. A number of very attractive effects can be produced with a little experimentation.
  
  
• It is also possible to introduce temperature fluctuations during the fire-down, in order to produce 'halos' in the crystals. Rapidly re-heating by a few tens of degrees, then proceeding with the fire-down as usual, will produce a halo. Multiple halos can be produced with multiple temperature variations; again there is much scope for experimentation.
  
  
• An optional soak at the top temperature affects the amount of crystallisation by giving the molten glaze longer to thin out. A longer soak gives less crystallisation. Introducing a soak of 10-30 minutes can be used if desired to reduce the density of crystal formation. Since crystal glazes are very fluid, the shape of the pot will also influence the density of crystallisation by altering the flow of the molten glaze: vertical sides produce fewer crystals than gently-sloped sides, since they cause faster glaze run-off.

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

1. Glaze not mixed and sieved thoroughly, or glaze has settled, so that glaze composition is incorrect.
   
   
2. Glaze applied too thinly.
   
   
3. Starting (upper) temperature of the fire-down was too low.
   
   
4. 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.
   
   
5. 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 than others.

No crystallisation occurs (glossy glaze)

1. Starting temperature of fire-down was very much too low.
   
   
2. Fire-down has been performed much too fast.
   
   
3. Glaze has settled or is not properly mixed.
   
   
4. Insufficient addition of Crystallisation Promoter for your clay body.

Pinholing occurs

1. Glaze may have absorbed alumina (see above).
   
   
2. 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

1. Excessively thick glaze application.
   
   
2. Excessively fast, forced drying of the glaze.
   
   
3. 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.

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 each glaze.
  
  
• 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.