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Glaze Terms

Glazes are the protective coatings that make ceramics safe and useful. Without the fired surfaces, the ware would not be able to hold water or be safe for food. The many types and textures of glazes that Mayco offers are almost endless - from bright, shiny reds to soft pastels, metallic gold to satiny, rich black. Mayco is committed to bringing the latest technology and safest products to market for the discerning consumer. All of Mayco colors are tested independently by a toxicologist to insure that they meet or exceed the government's strictest standards.

There are some terms that the consumer needs to know when choosing a glaze for any finished piece. These terms let the user know the characteristics of the glaze - whether it is shiny or dull, transparent or opaque, safe to use on utility items or for decorative items only. Reading the label is the best way to insure that you have selected the correct product for your technique and piece.

The jar label should be read each time that you use a product. At times raw materials may change or become obsolete. In order to continue producing a specific color, changes to the formula may be required. The label will have the most up to date information concerning your safety and that of the piece you produce.

Surface: The first category for glazes deals with the surface shine. The following terms will help you understand the labels more clearly and will allow you to make the best choice of products for great looking results. Some glazes fall in between all of these groups. Some glazes even have tiny specks or granules of a contrasting or complimenting color floating in them.

Gloss- Shiny and smooth, bright high gloss and a highly reflective surface.
Satin- Not really shiny, but not dull either. Think of an eggshell or a satiny sheen.
Matte- The opposite of gloss. This surface is duller than satin, reflecting little shine.
Dead Matte- Even duller than matte. This surface reflects no light.
Opacity- Refers to the transparency of the glaze. Some glazes are so transparent that it's like placing a clear piece of glass on top of the ware. These clear glazes can act like a magnifying glass on top of the ceramic surface or on top of underglazes. Some glaze colors are nearly opaque and others totally so.


C: Clear- The glaze is completely clear, adding only a shiny, wet look and bringing out the true underlying colors on the piece.
T: Transparent- The color underneath is slightly tinted by the overlying glaze changing the appearance only a little.
ST: Semi-Transparent- The underglazes beneath these are identifiable, but are changed by the tone of the glaze over them.
SO: Semi-Opaque- Light underglaze colors will not show through and dark colors will be muted.
O: Opaque- Most colors will not show through.

SAFETY: One of the most important considerations when choosing a glaze deals with their use of utilitarian or serving pieces. In our industry, several words are used to describe the level of potential hazard that a glaze poses to a customer. Some words are used to describe the product in the liquid state and others are used to describe the finished glaze surface after firing. To determine the toxicity of a glaze in its liquid state, formulas are submitted to an independently licensed toxicologist who examines each glaze formula and determines whether the product can be labeled as non-toxic or whether it requires a health caution label.

To determine dinnerware safe, a second set of tests are required. Samples of the glaze in its fired form (we use coffee cups) are tested by an independent laboratory facility for leachability of lead and cadmium. If the surface passes the standards set by the Food and Drug Administration (FDA), then it may be used on food contact areas and can be labeled as dinnerware safe. Dinnerware safe only refers to the leachability of lead and cadmium; other surface characteristics are not considered. Please be aware that if a finished surface is bumpy, grooved, pitted, cracked or rough, there is potential for bacteria to hide out in the surface texture. So, even if a glaze is technically dinnerware safe, it may not be practical for use on food or beverage containers due to the difficulty of cleaning. A comparison would be a cutting board and the care required when preparing foods.

Glazes fall into the following categories:

Non-Toxic: Refers to the product in the jar. Contains no harmful ingredients in sufficient quantities that could be harmful to humans (including children).
Health Caution: Refers to the product in the jar. There are some ingredients present in large enough quantities that the product may be harmful to humans. There will be detailed information on the product label as to the type of risk that is posed as well as proper handling instructions. Even more detailed information is available on the relevant Material Safety Data Sheet (MSDS).
Dinnerware Safe/ Food Safe: Refers to the finished, fired surface of the glaze. Once fired according to the instructions on the product label, the fired surface may be used in contact with food or beverage without leaching potential harmful elements from the glaze into the food or beverage.
Not for Dinnerware Use/ Not for Food Use: Refers to the finished, fired surface of the glaze. Even when the glaze is fired according to label directions, the fired glaze may potentially transfer harmful elements into food or beverage upon contact.

Here are some examples to illustrate the differences:

Stroke & Coat glazes are Non-Toxic and Dinnerware-Safe. There are no toxic elements present in the liquid state, and therefore, no toxic elements will be leached into food or beverage.
Classic Crackles and Crystalites are Non-Toxic, just like the Stroke & Coats. And although they are also Dinnerware-Safe, the surface should be examined to determine if it is easily cleaned after food contact (no bumps, cracks, grooves, etc.).
C-109 Wonder Clear Dipping Glaze has a Health Caution and is Dinnerware-safe. There is lead present in the liquid glaze and care must be taken to avoid exposure through inhalation or ingestion. However, when fired according to label directions, the fired surface of C-109 conforms to the standards established by the FDA and is dinnerware-safe.
Other products, like Exotic Glazes, have a Health Caution and are NOT for dinnerware use. There are potentially harmful elements present in the liquid glazes and instructions must be followed for proper use to avoid exposure. Even when the glazes are fired according to label instructions, they should not be used on food or beverage contact areas, as enough of the harmful elements may be transferred to the food or beverage to be considered harmful to humans.

Additional Terms

Antiquing: A method of applying color and wiping it back to accentuate the detailed surface.

Banding Wheel: A turntable operated by hand, used for decorating purposes.

Basecoat: Generally an all over coat of color on bisque on top of which detailed decorating is done.

Bat: A plaster disk or slab for clay work.

Bisque: Clay that has been fired but not glazed. Sometimes referred to as "biscuit."

Cadmium: Heavy metal used in producing red glazes and underglazes.

Casting: A clay form made from a mold. May also refer to plaster castings.

Casting Slip: A liquid clay used in the process of forming objects with molds. Also referred to as "slip."

Cavity (of a mold): The inside section of a mold where the casting is formed.

Ceramics: Clay forms which are fired in a kiln.

China: A term which usually refers to the bone china of England, but also is associated with vitreous white wares and porcelain.

Cleaning Greenware: The process by which mold seam lines and surface imperfections are removed from unfired clay objects.

Coats: Applications of ceramic color by brush, sponge, and spray which cover an entire area or a specific area of a piece.

Conditioned Brush: A brush lightly coated or dampened with a fluid to prepare it for the application of a specific type of color or medium.

Conditioning Coat (glaze or underglaze): A thin coat of color that will soak into the greenware or bisque well.

Cone (pyrometric cone): A mixture of ceramic materials that is designed to soften and bend when the proper mixture of time and temperature is reached in the interior of the kiln.

Cone Plaque: A small clay cone holder used when cones are placed on the shelf of the kiln.

Cone Temperature: The mixture of time and temperature at which the cone will bend.

Craters: Bubbles in the glaze finish which break.

Crazing: Tiny cracks that appear in the fired glazed surface.

Crawling: Glaze which pulls together and beads up, leaving bare spots of bisque. Also referred to as "separation."

Defloculant: The alkaline substance which is added in extremely small amounts to slip to make it more fluid without adding excessive amounts of water.

Dipping: The process of dipping ware into glazes.

Dry-Brushing: A technique of applying color which produces a feather-like effect using a dry brush and liquid colors.

Dryfooting: The process by which glaze is removed from the bottom or foot of a clay object so it may be fired without stilting.

Earthenware: Porous clay bodies which are fired to maturity at approx. 2000°F.

Element: The heating coils of an electric kiln. (Tired or burned-out elements refer to elements which carry too little or no electrical current for producing heat).

Fettling Lines (seam lines): The ridges created on a casting where the mold comes together. They are usually removed during the cleaning process.

Finger-sand: Gentle rubbing of the glazed surface to remove ridges.

Firing: The process by which ceramic ware is heated in a kiln to bring glaze or clay to maturity.

Fired Finish: A finish that must be fired to produce proper color and surface finish.

Fired Products (fired color products): Products which must be fired.

Firing Chamber: The interior of a kiln in which the ceramic ware is fired (also referred to as a fire box).

Firing Cycle: A system of gradually raising and lowering the temperature of a kiln to properly fire ware.

Flowing Coats: An application of glaze applied with a fully-loaded brush so the color flows onto the surface of the ware.

Food-safe: A product that has been tested and determined to be safe for use on surfaces which come in contact with food or drink.

Foot: The base or the part of the piece of ceramic on which it rests.

Glaze: A fired glassy coating on a piece of ceramic.

Gloss (G): A shiny, glass-like finish.

Greenware: Unfired clay forms or shapes.

Grit Cloth: A rough scrubbing material used in the process of cleaning greenware.

Hard Spot: An area on greenware or bisque surface that resists color application.

Hot Spot: A section of a kiln that fires to a hotter temperature than the rest of the kiln.

Incise: The process of carving a design into a greenware surface.

Keys (of a mold): The series of notches and bumps carved in the excess plaster around the cavity of the mold to insure a proper fit.

Kiln: The device in which clay and glazed clay objects are heated to maturity.

Kiln Furniture: The series of posts, stilts, and shelves on which the ceramic ware rests in order to take full advantage of the interior space of the kiln.

Kiln Sitter (automatic shut-off): A device used with a pyrometric cone to shut off the kiln when conditions inside the kiln cause the cone to bend.

Kiln Wash: The refractory coating applied to the top of the kiln shelves to protect them from glaze drips.

Lead Release: The amount of lead that is dissolved from the surface of a glaze which has been in contact with acid solutions.

Leather-hard: Greenware that is taken from a mold and is allowed to become firm but still retains its wet look.

Matte (M): A soft finish with little or no shine.

Matured Bisque: A bisque that has been fired at the proper rate of heating and cooling to produce an even state of hardness throughout.

Mini Bars: Pyrometric cones used to measure the firing temperature of a kiln. They are shaped like bars rather than cone-shaped.

Mold Strap (mold bands): Devices made of cloth, rubber, or metal used to tightly secure parts of a mold together during the pouring process.

Nesting: The procedure of stacking greenware in a kiln during the bisque firing.

Non-fired Finish: A color that is applied to bisque. These colors are never fired in a kiln.

OK Dinnerware: A product that when applied and fired according to label directions is safe for use on surfaces that come in contact with food.

Opaque (O): Color which does not allow other colors to show through.

One-Piece Mold: A mold that is made up of only one section or piece of plaster. Also referred to as an open-pour mold because of the lack of a pouring gate.

Peep Holes (vent holes): Small holes in the side of a kiln used for viewing shelf cones and ventilating the kiln during the firing process.

Pin Holes: Tiny holes in the final surface finish of a glaze or underglaze.

Plasticity: Refers to the ability of clay to be formed into a shape and retain it.

Posts (kiln): Articles made of refractory material which support kiln shelves during firing.

Pouring Spare: The excess clay formed at the pouring hole of a mold during the casting process.

Pour Hole (pour gate): A section at the opening of the mold used for pouring the slip into the mold cavity.

Prop: (1 ) A device of clay or refractory material used for supporting g greenware (usually porcelain) during the firing process. (2) Term applied to the slight opening of the kiln cover during the first stages of the firing process, kiln prop. (3) Another word for kiln posts.

Pyrometric Cone: A small piece of clay compound that reacts to time and temperature used to indicate maturity of ceramic clays and glazes.

Refractory Material: Substances that have a resistance to high temperatures.

Seam Lines: Small lines on greenware produced where two sections of a mold are locked together during the pouring process. Also referred to as fettlings.

Semi-Matt (SM): A satin-like surface which has a slight sheen to it.

Semi-Opaque (SO): Colors which generally allow only dark colors to show through.

Semi-Transparent (ST): Slightly colored and/or speckled colors which allow most colors to show through with only slight distortions.

Separation: See crawling.

Shelf Cone Temperature: The cone temperature that is fired on the shelf of a kiln. The amount of heating the ware actually receives.

Slip: See casting slip.

Soaking Cycle: A short cycle at the end of the regular firing cycle which maintains the level of heating in the kiln, and enhances many glaze finishes.

Spray Gun: See airbrush.

Sponge: Usually refers to a natural sponge used for cleaning and decorating greenware. There are also synthetic sponges available for ceramics.

Stain: Unfired colors used for decorating.

Stilts: Small shapes of bisque with metal or wire spurs used for supporting glazed greenware during firing.

Stoneware: A combination of clays which form a stone-like vitreous body during firing.

Test Cone Plaque: See cone plaque.

Three-piece Mold: A mold that has three pieces.

Thermal Expansion: The expansion that occurs in glazes and clays when heated in a kiln.

Thermal Shock: Sudden changes which occur in a clay or glaze which causes damage, usually through sudden heating or cooling.

Transparent (T): Clear base colors which are free from cloudiness or distortion.

Two-piece Mold: A mold that has two parts.

Underglaze: A color which is usually applied to greenware and in most cases is covered with a glaze.

Underfiring: Not firing hot enough or long enough, or both.

Vitrified: Usually refers to porcelain and stoneware that are fired at a high temperature. The clay begins to become glass-like in nature, although not necessarily waterproof.

Vent Holes: Small holes made by puncturing the wet greenware with a needle tool when two pieces of ware have been attached. These small holes allow the air and gases to escape during firing. Also refers to the peep holes in the side of the kiln.

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