Mixing Powder Glazes
Glazes supplied in powder form are mixed with water to make “slop” for dip glazing. Every glaze has different characteristics which may affect the amount of water required but as a starting point 1kg of lead free glaze powder needs approximately 500ml water and 1kg low sol (lead containing) glaze needs approximately 450ml water. The glaze powder should be added to the water rather than the other way round to avoid the glaze forming lumps in the mix. Wear a mask to protect against dust when adding the powder. It is also advisable to wear protective gloves while in contact with the glaze. Hold back about a quarter of the water to be added later to avoid making the mix too thin. Use a glaze mixer or paint mixer attachment to a power drill for best results, but avoid whisking air into the mix. Once mixed, sieve the glaze through a 120’s mesh sieve for earthenware or an 80’s mesh for stoneware. Glazes mix more thoroughly if left overnight to ensure all the powder is soaked. Clear water will form on the top as the glaze settles, pour off some of this before mixing, this can be added back if required when the glaze is mixed again and tested. Glaze is usually the consistency of single cream, a quick way to check is to see how it runs off your finger nails; it should initially cover the nail for a few moments, not run off immediately. A more accurate way to test the glaze is to dip a piece of the bisque and when dry check the thickness by scraping away the edge and measuring it against the thickness of a credit card. Fire some test pieces to be certain that the mix is correct before going into batch production. The porosity of your bisque will greatly affect the amount of glaze taken up in dipping; very porous bisque may need thinner glaze slop. In general terms transparent glazes tend to be thinner than opaque glazes. Glazes for spraying are generally thinner. When the glaze is perfect record the weight of a specific quantity, this will give you the density (often referred to as the pint weight). Before using always mix the glaze thoroughly as it will drop out of suspension, always check that there is not a layer of sludge in the bottom of the container. To aid suspension, you can add a suspending agent, proprietary brands are available and are used at about 10ml to 5ltr; calcium chloride or bentonite are also effective. Glaze hardeners such SCMC can be added to the mix to give a less powdery finish to the dry glaze. This is especially useful if the ware is going to be decorated with in-glaze colours (Majolica technique) before firing.
For more advice when mixing Potterycrafts glazes please call our technical department.