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Some notes on clays and firing

All our clay bodies are in plastic form and most sold in 12.5Kg bags, apart from paper clay which comes in 5Kg bags and Southern ice which comes in 9.25Kg bags. When stored in a cool but frost free environment, free from draughts and direct sunlight, the clays will retain their condition indefinitely.

Glaze suitability/Firing temperatures:

We recommend biscuit firing at 1000-1060 deg.C followed by a glaze firing at a higher temperature which matures both the clay and the glaze at the same time.

Each clay and each glaze is listed with its firing range. When selecting a glaze for use with a particular clay it is important to select one with a firing range above the lower temperature given for the clay. If this is not done the clay will be under fired and the glaze may craze.

Alternatively you could biscuit fire the clay above the lower figure given for the clay then glaze fire to the firing range of the selected glaze, however this method could cause difficulty with glaze pick - up when dipping although this can be overcome by adding Magnesium  Sulphate to the glaze which assists suspension and application to relatively vitreous biscuit ware.

Firing rate:

The optimum rate depends on the thickness of the pots and clay type e.g thin cast and grogged bodies can be fired quicker than thrown and and ungrogged ones. Ideally,biscuit firings would be slower than 100 deg.C per hour up to200 deg.C and 100-200 deg.C per hour beyond that point and ending in a short soak.

Glaze firings should be at 100 deg.C -130 deg.C per hour but at 100 deg.C per hour maximum from 700 - 1100 deg.C with high carbon ball clay or fire clay based bodies.If clays are fired  above their vitrification point then bloating is likely.

Bloating may also result from insufficient biscuit firing or too fast firing in the later stages of glost firing. Over-long firings may give excess heat work and cause bloating.

BPS Stoneware Glazes

For some years now we have been supplying our stoneware glazes in a ready mixed slop form. However despite the obvious advantage of having them in ready mixed, we do run into some problems when it comes to posting them! There is also, of course, the extra postal charge just for the water. With this in mind  we have experimented with tumble mixing the dry ingredients. The tests seem to be OK and so we are now offering BPS glazes in a powdered form too.

MIXING POWDERED GLAZES.

The glazes do need wet sieving through either an 80 or 100 mesh sieve and because the amount of clay used in each glaze varies you do have to use different amounts of water for each one. Here is our suggested water: powder ratio but please remember this is only a guide. The porosity of your bisque fired clay, the time taken to dip the work, whether or not you are spraying or double dipping and the firing temperature are all variable  factors to be taken into account. In some of the mixes there is some streakiness or specking in the powder e.g. iron oxide, but this is dispersed by mixing/sieving. You may also find that a few of the glazes leave a small amount of residue on the sieve. This has been allowed for in the recipe and can be discarded.

Ref no.BP

GLAZE SUSPENSION. 

The glazes that have a low clay content in the recipe ( and consequently are the ones that need less water in the mix especially BP5,8,9,11,& 13SP ) do tend to settle more quickly. We find that adding a small amount of calcium chloride liquid ( one or two tablespoonfuls per gallon ) is helpful. Do be careful however, not to overdo the calcium chloride as this can lead to a fault known as ‘plucking’ in which the glaze cracks and pulls away from the body during firing. Do NOT use the calcium chloride with the BP16P Orange Red, or BP18P Tenmoku. These both contain bone ash and the calcium Chloride reacts with it to form a very hard layer at the bottom of the bucket!

FIRING TEMPERATURE. 

This too will obviously affect the look of the glaze i.e. BP5,8,9,11,13&26P, will go shinier and become more transparent and ‘lose’ their speckled/variegated colours the higher you fire. If applied too thickly they will tend to run. These glazes are the most sensitive to temperature and will be satiny at cone 7, show a combination of shiny and satin specks at cone 8 and become increasingly clear and shiny as one goes up to cone 9 and above.

The cooling rate of the kiln ( which isn’t always under our control ) will also particularly affect these glazes - with a slower cooling allowing crystals to grow and giving a more satiny look. The Tenmoku BP18SP and the Oil Spot BP25SP seem better at the higher temperature although as a general rule they will become shinier and/or darker the higher one fires.

MEASURING TEMPERATURE AND FIRING ATMOSPHERE
The temperatures noted here refer to the heat work needed to bend a specific cone. As a general rule one often has to set an automatic electronic controller to cut off at a slightly lower temperature. For example Cone 8 is usually quoted as a 1260deg.C. cone but one often only needs to set the controller at around 1240deg.C. to get a Cone 8 to bend. The best practice is to put cones in the firing so that the temperatures set on the controller can be compared with the amount of bending on the cones.

Most of the glazes work well in reduction but the glazes BP9,13 and 23, which contain copper, will often come out reddish brown or various shades of pink! Instead of green. BP16, 18 and 25, which are very high in iron, will tend to go a metallic, iron red or purple.

Firing notes on lustres and enamels

Enamels: The usual temperature for firing the enamels is cone 017 to cone 016 (770
deg.C.- 790 deg C ) The reds pinks and yellow, however do have a tendency to fade out and become colourless. It is obviously advisable to fire these at the lower temperature around cone 017.

Lustres:  These are usually fired at cone 018 to cone 016 (720 deg.C.  – 790 deg.C.) especially when applied over earthenware glazes. However there can be a problem when using the lustres over high fired stoneware glaze when there is a tendency for the lustre to rub off. In this case one will have to fire a bit higher to soften the glaze, but not so high that it starts to sinter (  the first stage in melting ) which results in a harsh, roughened surface. The highest we have had to go is cone 013 (860 deg C.)

In this case it was gold FE3 and blue lustre on a satin matt stoneware glaze. This is unfortunately a matter of trial and error.

A word of warning. Larger plates and dishes are susceptible to thermal shock so that a gentle start to enamel / lustre firing may be needed.

Useful information on velvet underglaze colours

LEAD FREE UNDERGLAZE COLOURS.

*= suitable for use to 1300 deg.C.     + = contains cadmium and lead frit

Underglaze colours may be applied to green ware or onto biscuit  using universal medium. If sufficient flux is added, colours and stains  can be made self glazing and matt colours  developed.

VELVET UNDERGLAZE COLOURS

Velvet Underglazes are vibrant, semi translucent, fluxed colours  which are very versatile and offer three alternative techniques:

1.When left unglazed and then fired 1000 deg.C - 1100degC, they give semi matt surfaces to simulate cloth and fur textures. With slightly higher firing they assume a slight sheen. Glazing is not essential allowing  permanent decoration of finely detailed modelling without having the detail rounded away by subsequent glazing.

2.When used in normal Underglaze fashion they can be glazed by dipping , spraying  , or a transparent brush on glaze can be used. The colours intensify and give glossy results.

3.They can be used  majolica fashion on the unfired glaze, the flux content of the colours giving more predictable results.

Velvet Underglazes fire true to colour as applied from the jar and can be mixed to give  to give many more different shades. They tolerate a wide firing range and can be used on porcelain and stoneware with the exception of orange and red. Velvets vitrify at about cone 4 - 6  and can be used on clay, Bisque or on the  glaze and can be applied by brushing, sponging or spraying.

Velvets 380-384 have a unique formula that makes them very versatile. They can be used in detailed design work as a Cone 06/05 underglaze or they can be fired up to Cone 6 and maintain their intensity and brightness.

Application Methods

Brushing- Keep brush fully loaded at all times. For solid coverage, as in backgrounds, apply 3 or 4 coats of colour, working in opposite directions with each layer of colour. Wait for water sheen to disappear between coats.

Sponging - Pour colour onto a glazed tile. Saturate a slightly dampened fine sponge with colour and lightly pat onto the green ware or bisque , allowing the first coat to dry thoroughly before applying the next. Solid back-grounds can be quickly achieved by using this method which eliminates the streaking  that often appears when using a brushing method.

Airbrushing - Dilute colour with water to the consistency required for a smooth spraying application. For solid colour coverage airbrush 3 to 4 coats, for shading, one coat is generally sufficient.

Antiquing - Dilute 2 parts colour with 1 part water, for an antiquing solution. Apply to bisque with as large a brush as possible. Allow to dry. Remove the colour from the large areas with a moist sponge, rinsing and turning the sponge  often to produce clear highlights - exposing the embossed design.

Useful information on frit analysis

Axner Pens

AXNER OVERGLAZE PENS

Use the Axner Overglaze Pens to draw over previously glazed and fired pottery. Re-fire the pot to cone 05 ( even if it was fired to a much higher temperature ) for a beautiful Overglaze relief.

The Axner Overglaze Pen may also be applied over pottery with no glaze at all for a spectacular finish. Like the Axner Underglaze Pen the Axner Overglaze Pen is simple and easy to use. They are easy to clean and difficult to clog. They are refillable

AXNER OVERGLAZE REFILLS

These are available in 2oz pots and will refill an Axner Overglaze pen twice.

AXNER UNDERGLAZE PENS

Axner Underglaze Pens are easy to use and are refillable. They come with a medium metal tip. They are difficult to clog and easy to clean, and are available in 27 vibrant colours.

Temp. Range. At Cone 06 - 04 all colours strong and vibrant.
At Cone 6 most colours remain strong and vibrant. Exceptions: Chartreuse, Light yellow, Pink, Lilac & Light Pink,  are weak. Greens, Peach, Sun Tan, Purple and Maroon change colour somewhat.

At Cone 10 Same as Cone 6 with the following exceptions: Chartreuse, Lilac, Pink, Peach and Light Pink completely disappear.