Product Usage and General Glaze Troubleshooting

700. Clear Gloss Glaze. This is an excellent lead free clear glaze which is compatible with most commercially available underglazes. It has good tolerance for a wide variety of low fire clay body. We recommend applying it thinly, 2 brush coats, as it may cloud when applied too heavily.

701-747. Opaque Gloss Glazes. We offer a wide range of colors including excellent bright primary colors. These glazes require 3 of 4 coats by brushing, depending on how heavily you apply them. In general, more even coverage can be achieved by alternating the direction of brush strokes by 90° to the previous coat. These glazes do not bleed together very much so they can usually be butted against each other without the colors running together.

801-818. Semi-transparent Gloss Glazes. These semi-transparent glazes get darker in appearance where they go on thicker, such as in places where they flow into the details on pieces. Generally, it is best to apply them thinly, 2 brush coats. The exception in this group is 801 which is an opaque white glaze with black speckles.

250. Opaque Satin Glazes. These satin glazes provide solid coverage and a beautiful satin finish. They require 3 coats by brushing. Satin glazes tend to be stiffer and do not move much in the firing, so an even application is helpful. They will also respond to the firing temperature, such that the hotter they are fired the smoother and glossier the surface will become.

271-276. Opaque Texture Glazes. These texture glazes are very matt and stiff. They can be given an extra rough surface by applying the final coat with a sponge. Although they are lead free and non-toxic, they cannot be considered food safe because the rough surface may tend to trap bacteria.

120-170. Specialty Glazes. The two crackle glazes (138 and 170) are lead free and non-toxic, but cannot be considered food safe because of the possibility of bacteria growth in the cracks. The two metallic glazes (144 and 145) both require lead frits and various metal oxides and are not food safe. Pewter is quite a stable formulation and performs well at cone 05. Bronze tends to be a somewhat sensitive glaze. It provides a beautiful golden bronze finish if the thickness and firing temperature are correct. It needs to be applied heavily, 4 brush coats, and it needs to be fired hotter than normal, cone 03. The two cadmium red and orange glazes, 122 and 120, also require lead frits and are not food safe. They are also sensitive to glaze thickness and firing temperature. If the glaze thickness is too thin the color may go silver. If the glaze is too thick or the temperature is too hot, small black spots may develop. They should not be fired hotter than cone 06. Red glazes also need extra air space around them. Give good clearance around these pieces in the kiln. Reds also benefit from a faster firing, 4-5 hours.

1100's. Mid-range Stoneware Glazes, Cone 4-6. The satins in this family behave in a similar manner to the low fire satin glazes, being a little stiffer. They are numbers 1121, 1122, 1123, 1124, 1125, 1126, and 1127.

The Reactives in this family are lead free, non-toxic. They are food safe as long as there is a continuous vitrified surface when fired, this includes a vitrified clay body surface also. They are formulated to react during the firing cycle which can produce very interesting and beautiful effects. The final appearance of the glaze is dependent on firing temperature, glaze thickness, evenness of application and the clay body being used. The same glaze can look quite different from samples shown on the color charts. Different results will be obtained in reduction. They are numbers 1115, 1117, 1129, 1140, 1141, 1142, 1143, 1144, 1145, 1155, 1156, 1170, 1171, 1173, 1174, 1175, 1177, 1178, 1181, 1182 and 1183. Many of these type glazes are enhanced by a "sloppy" application. Overlapping is particularly fun with these glazes. The two or three glazes overlapped will create a mottled, variegated appearance even in oxidation.

Other reactives in this family include some glazes that are lead free, but they contain oxides of various heavy metal, such as copper, which exceed the toxic threshold levels in their unfired states. 1111 and 1159 are examples of saturated copper content glaze. (HL) label applies.

1200's. Hi-Fire cone 9/10 Glazes. Are not sold by KPS.

501-566. Underglazes. Spectrum offers a full palette of rich, vibrant underglaze colors that are opaque with 3 coats, semi-opaque with 2 coats and semi-transparent with 1 coat. They are compatible for use under most clear glazes. We suggest that you try them with one of our clear glazes (700, 250, 1100, 1120 or Amaco). Unlike many other brands of underglazes, Spectrum uses a frit base rather than a slip (clay) base for their underglazes. The advantages to the user are that they can be applied on greenware or bisque and they can be used with or without a clear glaze covering. When used without a clear glaze cover they have a matt velvet-like surface. The color charts at the end of the catalog show the unglazed color in the upper left half of the tile and the color when covered by a clear gloss glaze in the lower right half of the tile.

600-635. One-Stroke Underglazes. These are concentrated underglazes which are intended to give opaque coverage with one brush stroke. They are ideal for painting on designs and can be used in the same manner as underglazes.

2000-2088. Ceramic Stains. Not to be confused with bisque stains which are not meant to to fired, these ceramic stains come in powder form and can be used to color glazes, underglazes, overglazes, engobes and as body stains. In general they have greater temperature stability than some other commercially available stains, such as Mason stains. They are suitable for use from the lowest temperature ranges to the highest, although colors may vary somewhat with firing temperature and clear cover glaze or base glaze used. For specific recommendations on each stain see the price chart on page 27. The end color charts show the results for the stains used in our underglaze base and fired to cone 06. On the reverse side of the sheet are the same chips fired to cone 6. The strength shown on these charts was achieved by mixing 100 grams of underglaze base (liquid weight) with 50 grams of ceramic stain (dry weight).

Glaze Troubleshooting

Pinholes: The most common of all glaze defects, pinholes are tiny holes in the glaze surface which penetrate all the way through to the clay body. They are caused by gasses escaping from the clay body during the firing cycle, after originating from tiny pieces of organic matter, which is present in the clay. It can also be caused by dusty bisqueware. In lowfire, it is recommended to bisque fire 2 cones hotter than it is glaze fired to. If the pieces are dusty, it is easiest to dunk the piece fully into a bucket of clean water and immediately glaze. Sometimes a longer bisque fire cycle with a 5-15 minute soak at the peak temperature helps. It is also good to do a very slow cool cycle catching the cool down with a 5 minute soak every 200°. Other remedies include: changing the peak temperature to one cone higher; a thinner glaze coating; dunking the bisque in water before glazing; using a glaze with more flux and decreasing the amount of zinc or rutile in the glaze if it is present.

Crazing: Glazes that have crazed show a fine pattern of cracks in the surface of the glaze. Sometimes they are easier to detect by breathing on the piece and "fogging" the glaze surface. Crazing is a result of a mismatch between the coefficient of expansion of the glaze and the clay body. When the glaze has too high a coefficient of expansion relative to the clay body, crazing will occur. In other words, they are shrinking at different rates with the clay body being too large. The solution is to reduce this difference in expansion. This can be done by changing to a lower expansion glaze or adding a relatively low expansion material such as silica to the existing glaze. Alternatively, you can switch to a higher expansion clay body. or change your bisque to one or two cones higher. Remember, not all glazes fit all clays. Be sure to ask for our advice, we are more than willing to take the time to help you.

Shivering: When a glaze shivers it cracks and pieces of the glaze peel right off the piece, often at the edges of the piece. This is another mismatch in expansion coefficients, but this is the opposite to crazing. In this case the glaze expansion is too low relative to the clay body. One solution is to increase the expansion of the glaze by adding a material such as a high expansion frit.

Crawling: When a glaze crawls or creeps it will tend to mound up and expose an area of bare clay. This often happens in corners where glaze has built up too heavily or has not flowed all the way into the corners. Glaze can crawl in firing due to the presence of dust, grease, finger prints or other dirt on the bisque. Too much tin oxide will cause crawling, also. Sometimes crawling can be a fault of the glaze chemicals being milled to finely. It can also be caused by shrinkage and the consequent cracking of the glaze as it dries before being fired. Also, when underglaze is applied too heavily it may lead to crawling of the covering layer of clear glaze. One general solution to crawling is to thin the glaze either by adding water or by applying less glaze.

Blisters: Glaze Blisters look like little craters, approximately 1/8" diameter. Possible causes of blistering are: too thick a layer of glaze; insufficient drying of the glazed piece prior to firing; or too dense a clay body which traps air in the piece. Sometimes lowering the bisque and slowing down the cool will heal over blisters.

Settling Out: This is the most common problem for glazes prior to firing, which may also result in firing problems. When a glaze settles out some of the heavier components of the glaze settle to the bottom of the container. If you try to use this glaze without thoroughly remixing you will be applying a glaze with key ingredients missing. A glaze stays in suspension due to the presence of various types of clays, such as bentonite, and or gums, such as CMC. One common cause of settling out is the addition of too much water to the glaze, which dilutes the effect of the suspending agents and allows some of the glaze ingredients to settle out. Another possibility is the growth of bacteria which will consume an organic gum, such as CMC, and will lead to loss of suspension. To prevent bacteria growth do not return used glaze, which has been poured out of the original container into a new batch. Storing glaze in a hot or sunny environment may also encourage bacteria growth. Freezing can also destroy the action of CMC, so brushing type glazes in particular should not be shipped by unheated freight in the winter when the possibility of freezing exists. If a glaze has settled out, but has not gone rock hard in the bottom of the container, it can be re-suspended by the careful addition of Epsom salts or our Dry Suspender/Binder. Epsom salts can be readily purchased in most drug stores. First you need to create a saturated solution of Epsom salts by dissolving them in a cup of warm water until no more will dissolve (super-saturated). Then add this solution slowly and carefully to the glaze while continuously stirring the glaze. It should require less than approximately one teaspoon of Epsom salt solution per gallon of glaze. The quantity will depend on the severity of the problem, but you can add to much.

Downdraft Vents and Their Effects on Glaze Problems: Glaze defects like pinholes, blisters and a few other small problems are very frequently helped by the "fresh air in-contaminated air out" feature of downdraft vent systems.

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