Na₂O (Sodium Oxide, Soda)

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-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 Alkali 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). Recommended for iron reds, chrome yellows and manganese violets.

-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). Interestingly 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.

A down side of high feldspar glazes: Crazing!

This reduction celadon is crazing. Why? High feldspar. Feldspar supplies the oxides K₂O and Na₂O, they contribute the brilliant gloss and great color but the price is very high thermal expansion. Scores 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 can harbor bacteria and customers 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 possibly tolerate 10% of that. That would reduce running, improve fit and increase durability. Failing that, the next step is to substitute some of the high-expansion KNaO, the flux, for the low-expansion MgO, that requires doing some glaze chemistry.

Frits melt so much better than raw materials

Feldspar and talc are both flux sources (glaze melters), they are common in all types of stoneware glazes. But their fluxing oxides, Na₂O and MgO, are locked in crystal structures that neither melt early or supply other oxides with which they like to interact. The pure feldspar is only beginning to soften at cone 6. Yet 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! The frits progressively soften, starting from low temperatures, both because they have been premelted and have significant boron content. In both, the Na₂O and MgO are free to impose themselves as fluxes, actively participating in the softening process.

Ceramic Oxide Periodic Table

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).

This feldspar melts by itself to be a glaze, but crazes badly

Pure MinSpar feldspar fired at cone 6 on Plainsman M370 porcelain. Although it is melting, the crazing is extreme! And expected. Feldspars contain a high percentage of K₂O and Na₂O (KNaO), these two oxides have the highest thermal expansion of any oxide. By far! Thus, glazes high in feldspar (e.g. 50%) are likely to craze. Using a little glaze chemistry, it is often possible to substitute some of the KNaO for another fluxing oxide having a lower thermal expansion.

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