F.A.Qs

Welcome to the POTTERYCRAFTS Frequently Asked Questions area. If we have not answered your query please contact us via our contact page, or call our technical advisors.

what is Batt Wash and how do I use Batt Wash? Battwash is a multi- purpose refractory mixture of very fine ground zirconia and clay. This blended material is dispersed in water to produce a thin slip of cream-like consistency. mix the batt wash adding it into water and creating a single cream consistency. A thin layer of this is then painted by brush, or roller. it is best for the first use to apply 3 consistent coats allowing time between each for them to dry fully before applying further layers. it is best applied in alternate directions to give best coverage, coat once, turning the shelf or brush direction by 90 degrees onto refractory kiln shelves (batts) and this reduces the risk of pottery sticking to the shelves during firing because of glaze runs or poorly wiped feet. This layer does not need to be fired and fixed in place before using the refractory batt, ware can be placed directly on to the unfired layer. If the layer is damaged by excessive glaze drips then the batt should be scraped clean and a new layer of batt wash applied.

How do I mix powdered glazes? Sieve your powdered glaze into water through a 60-80s mesh sieve. For unleaded glazes, mix 100g of powder with 50ml of water. For leaded glazes, mix 100g of power with 45ml of water.

Do powdered glazes give the same quality finish as prepared glazes?Yes, if they have been mixed to the correct consistency.

Why is my earthenware glaze crazing? Mismatch of the thermal expansions of glaze & body (insufficient glaze compression), the glaze may not be suitable for the particular clay. In simple terms, when the pot has cooled and contracted after the firing the glaze is smaller than the clay body and is unable to sustain the tension put upon it. Often this is caused by underfired bisque so it is important to check the temperature of your bisque firing.

What is the difference between a stain and an underglaze & on-glaze colours? Stains are used to colour clay and glaze and are very strong colours. Underglaze colour is finer ground glaze stain and normally contains a frit or flux addition. Underglaze colours may also be used to colour glazes and clays but are primarily designed for applying to ware, either greenware or bisque which is subsequently glazed usually with a transparent glaze. They can also be used on top of a white glaze before firing, (Majolica technique). On-glaze colours are fusible colour where all the constituents are melted together and finely ground, they are applied on top of a fired glaze and are typically fired to 700°C/800°C. They are also called china paints or enamels.

How fast should I fire? The first firing, (bisque or biscuit) should be quite slow to avoid moisture trapped in the clay from turning to steam which will shatter the ware. If the controller has more than two heating ramps the best approach is to fire to around 80°C at a rate of 80°C per hour and then soak at that temperature for one hour. The second ramp should then be 100°C per hour up to 500°C the third heating ramp can be faster, 150 or 200°C per hour to the top temperature. If there are only two ramps make the first around 70°C. Fire even slower for large pieces. Glaze firing can usually be at a faster rate; typically 120/150°C per hour is acceptable. Make the first set point no lower than 600°C, after this temperature it is safe to go faster 150/250°C to top temperature. Larger pieces should be fired more slowly.

What is the difference between Earthenware & Stoneware? Earthenware is fired at lower temperatures usually not more than 1180°C. At this temperature the clay remains porous and the glaze will be a separate layer adhering to the surface. Stoneware is fired to higher temperatures, maturing the clay and glaze at the same time. The glaze interacts with the clay forming an integral glaze/clay layer. Stoneware biscuit firings are usually around 1000°c and glaze firings 1230°C to 1300°C.

Can I use Earthenware brush-on glazes on Stoneware clays? If stoneware clay is fired to maturing temperature it will be difficult to apply brush-on glazes as the surface will not be porous, if this can be achieved then the earthenware glaze might fire successfully but there is a risk of crazing. The glaze must be fired to its specified temperature. Some stoneware clays are dual purpose and can be used at earthenware temperatures.

What raw materials and oxides make a glaze not food safe? Where toxic materials are used in Potterycrafts glazes they are present in a safe form where they are chemically bonded in "fritting" or "encapsulation" processes. These processes render the substances "not bio-available" and therefore safe to use in making and after firing. For example where lead is present in "low sol" or fritted form the Health & safety Executive do not require it to be to labelled toxic. If fired correctly most proprietary glazes will be food safe. Glazes with a textured, crazed or a very matt surface may not be food safe as the surface can harbour bacteria. If in doubt about a specific glaze, or if you have concerns please contact our technical advice department. For potters mixing their own glazes the following substances are considered toxic however most of them are no longer available except in fritted or encapsulated form: Lead, Cadmium, Selenium, Arsenic, Antimony, Barium, Zinc, Nickel, Cobalt.

Can I mix glazes (i.e. yellow and blue to make green)? Some glaze ranges such as Amaco Teachers Palette are specifically designed to be intermixable; other glazes might mix but would not necessarily achieve an intermediate colour. Mixing glazes could alter qualities other than colour, texture and firing temperatures could be affected; always test before going into production.

Do I need 3 phase electrical supply to run a front loading kiln? It is usually possible to run any of our kilns, top or front loader up to 13kilowatt size from a domestic 100 amp single phase supply. This would give you a kiln up to about 8 cubic foot 225 litres. At 13kW you would require 60amp supply which is the maximum recommended for a single appliance running off a 100amp supply. It is essential to have your supply checked by a qualified electrician before purchasing a new kiln, if in doubt we can speak directly to your electrician.

What is Pinholing? Small round, smooth edged holes in the glaze surface can be caused by gases from impurities in underfired bisque or slightly underfired glaze. In re-fired ware such as china painting it is caused by moisture absorbed into the body being forced out during the firing.

What is Thermal Shock? When ware is heated or cooled unevenly stresses develop causing cracks to occur, this is because of different rates of expansion or contraction within the piece. Heating and cooling more slowly reduces the temperature differential as heat is conducted through the item.

What is the difference between Tin and Zircon opacified glazes? Tin glazes are blue/white in colour, offer a broader firing range than Zircon glazes and require approximately half the oxide addition when compared with Zircon.

Why does my Kiln smell during the first stages of the firing? Organic, vegetable matter present in clays and suspending agents and binders in glaze start to burn away at quite low temperatures and can produce strong smelling fumes. This will die away as the kiln reaches 500/600°C. With most firings the level of toxicity in fumes is very low, on a par with fumes present from traffic in a High street; however working in a room where a kiln is firing is not recommended. If you would like advice on kiln ventilation please call.

At what temperature should I fire the kiln to for the first firing and following element replacement? New elements fired to around 1100°C in an empty kiln will develop the oxide layer on the surface of the wire more quickly and should improve element life. If the glaze or clay being used produces aggressive fumes such as fluorine then this is worth doing, with usual materials the benefit is marginal.

At what temperature can I open my Kiln after the firing cycle? If the door is opened when the kiln is above 200°C there is a risk of cracking the pottery because of uneven cooling at the critical point of sudden contraction. This is due to crystobalite inversion, a chemical change occurring between 280°C and 220°C. Although safer for the pottery below 200°C, beware of a risk of burning until the kiln has cooled to room temperature.

An element has come out of its groove - what should I do? Moving or bending an element can result in it snapping as they become very brittle with firing. First heat the element to red heat with a blow lamp and then bend to shape with long nosed pliers. Staples made of element wire might be helpful to hold it more securely.

The bricks on my kiln are cracking? New designs, materials and methods of construction are minimising this problem however some cracking may occur because of expansion/contraction stresses. Opening the kiln too early will increase the risk of bricks cracking. As long as they are not structural even severe cracking will not affect the firing performance.

Can I fire the Kiln unattended? Potterycrafts latest kilns all have over-temperature and anti power surge devices which make them safe to fire but we advise that there should be some supervision during the firing especially at the end when the kiln completes its firing.