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.
  
  
• Batches of glaze manufactured after November 2001 also include a sachet of Crystallisation Promoter (fine white powder, labelled P). This ingredient, which was already mixed-in in earlier batches of glaze, 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. In most cases you will probably wish to add some or all of the Promoter, but when working with clays which leach a lot of alumina into the glaze (semi-porcelain or stoneware, for example) you might find that sufficient crystallisation occurs with no Promoter at all, and adding the Promoter will cause a solid mass of crystals.
  
  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 the Promoter sometimes causes solid masses of interlocking crystals. If this effect is not desired, but isolated large crystals are preferred, the glaze may function best with the Promoter entirely omitted or added very sparingly. 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.
  
  Experimentation is the best method to explore the possibilities of these glazes on your particular clay body, if you are unsure about its performance. It is simple to mix up the glaze without any Promoter, then add the Promoter stepwise, testing the glaze at each addition of Promoter until you find the addition that gives the type of crystals that you prefer.
  
  
• 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. The Dawnmist Studio range of casting slips includes both translucent porcelain (NW808) and a semi-porcelain (NW804) which gives excellent results without the difficulties associated with true porcelain and with brighter colours than are usual on porcelain. A very smooth, ungrogged white stoneware (such as NW801 casting slip, or Earthstone Extra Smooth plastic body) 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.
  
  
• Crystalline glazes are by nature very fluid, 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'. Dawnmist SuperStilts, rated to Cone 9, are usually suitable; these are designed to be easy to separate from fired pots. A 'catch bowl' is typically a crudely-made shallow bowl a little larger than the diameter of the pot, into which the stilt is placed. 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 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 a one-use catchbowl/stilt combination, 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' (available from Dawnmist Studio) into the soft clay so that they support the base of the pot. Then bisque 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 the risk of toppling.
  
  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.
  
  
• Any blobs of glaze left on the bottoms of pots can be ground down with a fine carborundum stone (manual or power-driven) after the stilts are removed; in fact 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.
  
  
• 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. 'Tide marks' 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), 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 polished-smooth (semi-)porcelain will be more likely to form individual large crystals, an unpolished bisque pot will be more likely to have 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.

• 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

  NW902/3/4/6/8/10 will be found to crystallise a little slower than NW901/5/7/9 due to the colourants used. NW906 is significantly slower than the others.
  
  

• 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.

This section lists problems that may be encountered, along with a list of likely causes and appropriate remedies.

Small, rough crystals or mattness

1. Glaze not mixed and sieved thoroughly, or glaze has settled.
   
   
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.

This is a risk with crystalline glazes as they are very low in clay and consequently do not always adhere well. They also exhibit greater drying shrinkage than most glazes because of their unusual composition. A number of approaches can be used besides the obvious one of thinning the glaze coating:

• Even quite large cracks do not matter. Since crystalline glazes are extremely fluid, they will 'heal' during firing, and will be most unlikely to crawl.
  
  
• 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.
  
  
• Add Acrylic Medium to the glaze as a plasticiser and adhesive. This material, typically used for preparing acrylic colours for screen-printing or fabric-painting, is obtainable from most art shops. It can also be supplied by Dawnmist Studio. A typical addition of 3--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.
  
  
• Dampen the bisque slightly before applying glaze.

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.

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The following statements are applicable as shown to these glazes:

1. HARMFUL by inhalation and if swallowed. Do not breathe dust.
2. DANGER of cumulative effects.
3. MAY CAUSE SENSITISATION by skin contact. Wear gloves, avoid skin contact.
4. Contains silica.
5. Contains cobalt compounds.
6. Contains barium compounds.
7. Wash hands thoroughly immediately after contact.
8. Avoid breathing dust.
9. TOXIC by inhalation and if swallowed.

Glaze	Applicable Statements
NW900 Seeding Agent	1, 2, 4
NW901 Ivory Pearl	1, 2, 4, 6, 7
NW902 Brown Sugar	1, 2, 4, 6, 7
NW903 Mint Green	1, 2, 4, 6, 7
NW904 Deepest Blue	1, 2, 3, 4, 5, 6, 7
NW905 Golden Beige	1, 2, 4, 6, 7, 9
NW906 Midnight	1, 2, 3, 4, 5, 6, 7
NW907 Sky Blue	1, 2, 3, 4, 5, 6, 7
NW908 Turquoise	1, 2, 3, 4, 5, 6, 7
NW909 Silver-Grey	1, 2, 3, 4, 5, 6, 7
NW910 Pastel Green	1, 2, 4, 6, 7

The flocculant enclosed with powder glazes NW901--NW910 is non-hazardous.

The Crystallisation Promoter enclosed with the glazes is subject to statements 1, 2 and 4.

ENQUIRIES TO ORDER OUR GLAZES WELCOME...

sales@dawnmist.org

Price and ordering information

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