|Co-efficient of Linear Expansion||0.387|
|Frit Softening Point||920C (From The Oxide Handbook. We also have a figure of 800C?)|
|Dry M.O.R. (50% Silica)||426C|
-Sodium is a slightly more powerful glaze flux than potassium, but otherwise very similar in its behavior and properties. Together they are referenced as KNaO. Sodium belongs to the Alkaline group. It is the strongest common flux and works across all temperature ranges from 900-1300C. Thus care must be taken to avoid excessive melt fluidity in glazes having significant sodium. Almost all frits and glazes have at least some KNaO.
-Sodium is sourced primarily from feldspars and frits. Some frits are available that have double the sodium content of feldspar yet very little Al2O3, most have at least some B2O3.
-Sodium produces bright and brilliant glaze surfaces and gives strong color responses to copper, cobalt, and iron (coupling high alkali with low alumina gives the most intense colors).
-High soda glazes can often be soluble and easily scratched, so other oxides are also needed (like CaO, MgO) to produce durability, tensile strength, elasticity and leach resistance. Bottle glass is made using a combination of soda and lime as the fluxes (float and container glasses have about 13% Na2O). Interesting these glasses have only 1-2% Al2O3, their durability is possible using the chemistry: 13% Na2O, 10% CaO and 75% SiO2.
-Unfortunately, the bright colors possible with high Na2O come at the expense of glaze fit. It has a higher thermal expansion than any other oxide and will promote crazing (especially in glazes lacking silica and/or alumina). Glazes having a high feldspar content (over 35-40%) are thus prime candidates for crazing. If a specific color effect requires high sodium (e.g. copper blue) it may be necessary to adjust the body to eliminate the crazing (increase its thermal expansion). Crystalline glazes, for example, are high in sodium and most often crazed. Celadons and copper reds also tend to be high in sodium, they likewise craze in the hands of many people. However it is possible to find a compromise between brilliant color and low enough thermal expansion if you substitute some of the KNaO for CaO, MgO, BaO, SrO or Li2O3 and maximize the SiO2 and Al2O3 (while still getting a good melt). Boron can also be employed, it has a very low expansion and enables adding more SiO2 and Al2O3, they push it down further. Some glazes have high sodium content (and thus craze) completely unnecessarily. Cone 10 dolomite mattes are an example, some have 60% feldspar! The mechanism of these is high MgO in an otherwise fairly fluid base. However MgO has the lowest thermal expansion of all fluxes and it works well even if SiO2 and Al2O3 are high. CaO is a very active flux also (with a much lower thermal expansion), it can easily handle the majority of the fluxing duties. Of course, you need a little glaze chemistry to do all of this.
-Soda works well with boric oxide (and also lithia and potassium) in low temperature lead-free glazes.
-The alkalis can increase lead solubility.
-A number of common sodium-sourcing materials are soluble (e.g. soda ash, borax) or slightly soluble (e.g. nepheline syenite, sodium frits, Gerstley Borate). Special techniques or considerations are required to use these materials in glazes.
-Sodium can begin to volatilize at high temperatures, this is the mechanism of soda and salt glazing.
All common traditional ceramic base glazes are made from only a dozen elements (plus oxygen). Materials decompose when glazes melt, sourcing these elements in oxide form. The kiln builds the glaze from these, it does not care what material sources what oxide (assuming, of course, that all materials do melt or dissolve completely into the melt to release those oxides). Each of these oxides contributes specific properties to the glass. So, you can look at a formula and make a good prediction of the properties of the fired glaze. And know what specific oxide to increase or decrease to move a property in a given direction (e.g. melting behavior, hardness, durability, thermal expansion, color, gloss, crystallization). And know about how they interact (affecting each other). This is powerful. And it is simpler than looking at glazes as recipes of hundreds of different materials (each sources multiple oxides so adjusting it affects multiple properties).
Feldspar and talc are both flux sources (glaze melters). But the fluxes (Na2O and MgO) within these materials need the right mix of other oxides with which to interact to vitrify or melt a mix. The feldspar does source other oxides for the Na2O to interact with, but lacks other fluxes and the proportions are not right, it is only beginning to soften at cone 6. The soda frit is already very active at cone 06! As high as cone 6, talc (the best source of MgO) shows no signs of melting activity at all. But a high MgO frit is melting beautifully at cone 06. While the frits are melting primarily because of the boron content, the Na2O and MgO have become active participants in the melting of a low temperature glass. In addition, the oxides exist in a glass matrix that is much easier to melt than the crystal matrix of the raw materials.
This reduction celadon is crazing. Why? High feldspar. Feldspar supplies the oxides K2O and Na2O, they contribute to brilliant gloss and great color (at all temperatures) but the price is very high thermal expansion. Any glaze having 40% or more feldspar should turn on a red light! Thousands of recipes being traded online are high-feldspar, some more than 50%! There are ways to tolerate the high expansion of KNaO, but the vast majority are crazing on all but high quartz bodies. Crazing is a plague for potters. Ware strength suffers dramatically, pieces leak, the glaze harbours bacteria, crazing invites customers to return pieces. The simplest fix is to transplant the color and opacity mechanism into a better transparent, one that fits your ware (in this glaze, for example, the mechanism is simply an iron addition). Fixing the recipe may also be practical. A 2:1 mix of silica:kaolin has the same Si:Al ratio as most glossy glazes, this glaze could likely tolerate a 20% addition of that quite easily. That would reduce running, improve fit and increase durability. If the crazing does not stop the next step is to substitute some of the high-expansion KNaO, the flux, for the low-expansion MgO, that requires doing some chemistry in your insight-live.com account.
|Oxides||KNaO - Potassium/Sodium Oxides|
|Oxides||K2O - Potassium Oxide|
Fluxes are the reason we can fire clay bodies and glazes in common kilns, they make glazes melt and bodies vitrify at lower temperatures.
In glaze chemistry, the oxide is the basic unit of formulas and analyses. Knowledge of what materials supply an oxide and of how it affects the fired glass or glaze is a key to control.
Crazed ceramic glazes have a network of cracks. Understanding the causes is the most practical way to solve it. 95% of the time the solution is to adjust the thermal expansion of the glaze.
Crackle glazes are used on decorative ceramic ware. They have a crack pattern that is a product of thermal expansion mismatch between body and glaze.
|Glaze Color||Oxidation copper blues work best in high alkaline, low alumina glazes. Increasing copper to 4-6% will move color toward turquoise.|
|Glaze Color||Copper red reduction glazes are best in formulations with high alkali. The presence of boron can give a more pleasant red.|