The center portion was covered and so it lagged behind during drying, setting up stresses that caused the disk to crack. This test is such that most pottery clays will exhibit a crack. The severity of the crack becomes a way to compare drying performances. Notice the test also shows soluble salts concentrating around the outer perimeter, they migrated there from the center section because it was not exposed to the air.

Low temperature clays are far more likely to have this issue. And if present, it is more likely to be unsightly. The salt-free specimens have 0.35% added barium carbonate.

Example of a custom made dust collection hood in the material repackaging area of a supplier. The slots along the front suck particles into the duct, the suction comes from an exhaust fan downstream where the pipe exits the building. It has a wall switch and a sliding damper (where the hood enters the pipe) to enable stopping all airflow (to prevent heat loss in the room during cold days). Notice it is located above the scale and heat sealer where most dust is generated during weighing and packaging. Working in front of a system like this enables you to work without breathing any dust at all.

This material storage area employs a rack to keep pails off the floor so the area can be hosed down easily. The materials in each pail are sealed in plastic bags or the pail is covered with a lid.

A cone 6 stoneware with 0.3% 60/80 mesh manganese granular (Plainsman M340). Fired from cone 4 (bottom) to cone 8 (top). It is normally stable to cone 8, with the manganese it begins to bloat at cone 7. The particles of manganese generate gases as they decompose and melt, these produce volumes and pressures sufficiently suddenly that closing channels within the maturing body are unable to vent them out.

Two bisqued terracotta mugs. The clay on the right has 0.35% added barium carbonate (it precipitates salts dissolved in the clay to prevent them coming to the surface with the water and being left there during drying). The process is called efflorescence and is the bane of the brick industry. The one on the left is the natural clay. The unsightly appearance is fingerprints from handling the piece in the leather-hard state, the salts have concentrated in these areas (the other piece was also handled, but has very little marking).

Like this! This terra cotta clay vitrifies here at 1957F (cone 03). This problem is common in many terra cotta materials but can also surface in others. Barium carbonate can be used to precipitate the salts inside the clay matrix so they do not come to the surface on drying.

Laguna Barnard Slip substitute fired at cone 03 with a Ferro Frit 3195 clear glaze. The very high bubble content is likely because they are adding manganese dioxide to match the MnO in the chemistry of Barnard (it gases alot during firing).

Causes eye irritation. Prolonged contact with skin may cause irritation.

This bag will give you a clue as to what manganese dioxide is mainly used for.

This designation is an international standard for a general purpose respirator to filter out respirable quartz particles (which cause silicosis). Use one of these when working in a area where ventilation is insufficient to remove all of the dust. Use it also in circumstances where there is temporary generation of large quantities of dust. Do not wear this as a substitute for keeping floors and working areas clean.

Black burning bodies are popular with many potters. They are normally manufactured by adding around 10% burnt or raw umber to an existing buff-burning cone 6 stoneware. Umbers are powerful colorants, they have high iron and also contain manganese (the latter being the primary source of the color). But these clays can be troublesome. First, good kiln venting is needed to avoid breathing the dangerous manganese metal vapors. Micro-bubble clouding/gloss-loss in the glazes and blistering/bloating of the bodies are common. But this mug fired perfectly. Why? The umber was added to a cone 10 stoneware instead (and it has fluxed the body to mature at cone 6). The mug has been white engobed on the inside and partway down the outside during leather hard stage. After bisque it was clear glazed on the inside giving a flawless surface (using G2926B) and dipped in GA6-A Alberta Slip base amber-clear. The GA6-A over the black clay produces a very deep, rich, almost black ultra-gloss surface.

This type of clay is made by a number of North American manufacturers. Generally, raw or burnt umber are used to stain the color (10% or more). Umber contains a high percentage of manganese. The umber also fluxes the body and some manufacturers make the mistake of adding it to a cone 6 body and this is the result.: bubbling of the glaze (in this case it is a transparent) and bloating and warping of the body. If the manganese is added to a cone 10 clay, the umber will flux it to mature around cone 6 without these problems. These bubbles are happening because the umber is decomposing and potentially gassing metal fumes. A functioning kiln vent is a must to fire this type of body.

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