NOTES ON STONEWARE GLAZES

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, wether 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

Water ccs.per kg

8,9,11,13 P

800

5P

950

2,16,18,19,21,22,23,25,27,31 P

1000

1,3,14,15,20,25,26,28,36,37 P

1100

4,17,24 p

1200

32,33,34,35 P

1750

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.