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  • Leaching, glaze solubility

    Glazes are not as inert and stable as most people think. All are slightly soluble and will thus leach into liquids they come into contact with. However some glazes are dramatically more soluble than others and lead glazes have gained this reputation. The subject of leaching and glaze safety is a complex one and ceramic calculations and chemistry can be used to great benefit in this area. The Magic of Fire book has much information on this subject.

  • Lead solubility, lead release in glazes

    Lead release in glazed ware is a critical thing to consider if you make ware at low temperatures and employ lead based materials. The entire ceramic industry has been forced to progressively move away from leaded glazes and colors over the past decade. It is important to realize that using lead frits does not make lead glazes safe, it just makes handling lead glazes safer. The solubility of the fired glaze is dependent on its chemistry and the way it is fired. If you do not know that formula for a frit you are using, check it out, do not assume it is lead free.
    Lead release is normally tested by measuring how much of the glaze will dissolve in a dilute acid mixture.
    Testing kits are available for $25 and come with a neutralizer (sodium hydrate) and an indicator (sodium sulfuret). Both have warnings about flushing skin or eyes thoroughly with water if splashed on and drinking 3 or 4 glasses of water or milk if ingested (do not induce vomiting).
    One set of instructions, for example, says to wash the object, immerse in white distilled vinegar (5% acetic acid - any vinegar will do) for 18 hours (or longer), stir the vinegar, fill the supplied test tube to the etched line (about 2 cc), add 7 drops of neutralizer, invert to mix. After 2 minutes, add 7 drops of indicator and invert to mix. After 2 minutes a clear or milky solution is OK, any shade of brown indicates lead according the comparison chart provided.
    Lab testing can be done at Elemental Research Lab, 309-267 West Esplanade, North Vancouver, B.C. Canada V7M 1A5 604-985-0445 (they test many metals). Also at Kirby Health Center Lab, 71 North Franklin Street, Wilkes-Barre, PA 18701 717-822-4278.

  • Lime Popping

    A body firing problem where particles of calcium sulphate in the fired or bisque fired body slowly absorb water from the air, expand, and pop out a section of the body.

  • Liner Glaze

    It is common to glaze food surfaces of utilitarian ware with a white or transparent glaze called a 'liner glaze'. This is done to avoid exposing in-glaze or on-glaze metallic colorants to food or drink (which could leach them away). Liner glazes can be applied in such a way that they meet the outer glaze at time rim. This is done by glazing the inside and lip first, then the outside using the colored glaze. More specifically, pour the glaze into the piece and then pour it out while turning it. Immediately press the rim into the liner glaze a few millimeters. Wax resist the glaze covered rim and down inside about a couple of centimeters. Cut away the glaze on the outer edge using a sharp knife at a 45 degree angle (thus leaving the inner liner glaze coming to the peak of the rim with a wax overlay). Finally, press rim into the colored glaze about 1 cm then quickly turn the piece over and press it down into the glaze up to the rim (taking care not to go over it). Then sponge off the glaze drips).
    Pictures
    An example of how a liner glaze can meet another glaze at the rim of a piece. This it quite simple to achieve.


    An example of three cone 10R mugs that have a liner glaze.


  • LOI

    Simplistically, LOI is the amount of weight a material loses on firing. LOI is usually crystal-bound water or carbon material that burns away.

    However LOI can also be other things (e.g. sulfur, chlorine, oxygen). It is also more complex for many materials since they go through a number of changes during temperature rise and these changes may involve weight loss or even weight gain (e.g. for non-oxide materials that capture oxygen from the atmosphere or other materials). Some of these changes may occur at a temperature higher than that at which they are being used. In addition, some materials that might otherwise break down quickly and form gases of decomposition may linger when they are in a glaze or frit that is being fired quickly (e.g. fluorine).

    LOI is important in ceramic chemistry, especially with INSIGHT. The primary purpose of recipe calculations is to derive the formula for the glass that comes out of the kiln, from the mix of recipe materials that go into the kiln. A fired glass has no organics or carbonates; so it always has zero LOI. This means that LOI is never shown for a glaze formula and you will never need to worry about it for any batch-to-formula or analysis calculations.

    However, many raw materials that go into the kiln do lose weight during firing; so they are not sourcing as many oxide molecules as a calculation might suggest. If a raw material loses weight on firing, it must be accounted for in calculations. You can think of LOI as being like the shells which we throw away from a bag of nuts. We compensate for anything lost during firing by increasing the formula weight. For example, 100 grams of kaolin going into a kiln produce only 88 grams of oxides for glass making. By increasing the formula weight of the kaolin by the correct amount, a full calculated oxide yield will result. The INSIGHT software stores a material’s formula in its MDT (materials database) exactly as you enter it. It requires a formula weight for each material; so when needed it can calculate the material’s LOI as the difference between the recorded weight and the actual sum of the weights of the oxides in the formula.

    Since INSIGHT knows the LOI for each material in a recipe, it can calculate the LOI of the raw recipe as a whole. This can be very useful. For example, if you are blending materials to create a composite material that will be used in recipes, you need to know its LOI when you add it to INSIGHT's materials database.

    If you have an analysis lacking an LOI figure or suspect the accuracy of the analysis delivered by a lab, then you can weigh, fire, and weigh again to derive the LOI and compensate the analysis. Following is mathematical method of applying a 5% measured LOI to an existing analysis. This is called "LOI Compensating an Analysis".
    100 - 95 = 5 / 100 = 0.95
    
--------------------------
    
K2O    7.3% x 0.95 =  6.9%
    
CaO    9.4% x 0.95 =  8.9%
    
MgO    1.0% x 0.95 =  1.0%
    
ZnO    1.0% x 0.95 =  1.0%
    
Al2O3 11.8% x 0.95 = 11.2%
    
SiO2  69.5% x 0.95 = 66.0%
    
LOI                   5.0%
    
--------------------------
    
     100.0%  100.0%

    Out Bound Links

    In Bound Links

    • (Glossary) Formula Weight

      Quite simply, the weight of a formula. Typically, ...

    • (Project) Stains

      We make no attempt to classify or compile stains a...

  • Low Temperature Glaze Recipes

    This term generally refers to glazes that mature from cone 06-04 and are not functional. Low temperatures require the use of boron-containing materials (e.g. frits) to produce a well melted glass. While low fire glazes are not as hard as well formulated medium temperature glazes, they have the advantage of supporting a much wider range or colors (which can be very bright and vivid). Historically Gerstley Borate and Colemanite have also been used to source boron in low fire glazes. Generally, at low temperatures the major challenges are to get a well melted and well fitted glaze (the clay glaze interface is not well developed and bodies are not vitreous so glazes must have compatible thermal expansion to the body to avoid coming off). The oxides commonly regarded as fluxes at middle and high fire are often simply fillers (and even matting agents) at low temperatures, thus understanding the relationship between chemistry and physical properties is more challenging.

    In Bound Links

  • Lustre Colors

    Common materials used in lustres are stannous chloride, barium chloride, barium chloride, sodium chloride, bismuth subnitrate, silver, gold, and platinum. We do not provide a definition here however you can look for these books:

    Ceramic Colours and Pottery Decoration by Kenneth Shaw, published by Maclaren and Sons Ltd., London, 1962, reissued 1968.
    Lustres by Margery Clinton, published by BT Basford Ltd. London, 1991. This book covers how to make and use lustres.

  • Lustres, lusters

    Lustres are very thin coatings of metallic substances fired at comparatively low temperatures onto an already fired glaze surface. They produce iridescent effects.


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