|Monthly Tech-Tip |
|Co-efficient of Linear Expansion||0.068|
|Frit Softening Point||723C|
-Together with sodium and potassium oxides, it is classified as one of the Alkali group.
-The lithium molecule is the lightest and smallest of the fluxes (it has a low molecular weight). And the most reactive. Adding small amounts by weight introduces disproportionately large molecular amounts to the glaze formula. Lithium-containing boron frits can melt as low as 1100F (600C) with a little help from Bi2O3 and F to go that low.
-Because it can produce such melt-fluid glazes very good depth of color is possible. Li2O is useful across a very wide range of temperatures by simply adjusting its percentage in the batch. One user said this: "It is easy to adapt but like trying to tame a wild animal. Hmmm, beautiful electric glazes or electric cars?"
-It is highly effective together with boron and sodium at lower temperatures.
-Lithium Carbonate, its main source, has a very low melting point as a material, it is a very active and powerful flux. And very expensive!
-In frits and glazes, lithia is used to reduce the viscosity and thereby increase the fluidity of the coatings. This reduces maturing times and lowers firing temperatures.
-1% additions can increase glaze gloss to a marked degree and slightly greater amounts (3%) can reduce the melting temperature by several cones and affect the surface tension of the melt.
-High cost limits its use in larger amounts, but in small amounts, it acts as a powerful auxiliary alkali flux with welcome thermal expansion-lowering effects. However, in large amounts lithia can drastically increase the thermal expansion of a glass.
-Calculated expansion projections tend to break down with all but low additions of lithium to glazes (less than 5%). Its contribution is nonlinear, especially in high sodium and potassium glazes. Often high lithium glazes appear to shiver whereas the calculated expansion does not indicate a sufficiently low expansion. It is known that molten lithia is mobile (diffuses into the surrounding matrix because of its small ionic radius and low charge). It can also diffuse into the body and create a low expansion glaze interface. One theory proposes that glazes with more than about 5 mol% Li2O could develop a lithium-rich interface (this could be coupled with a lithium-deficient upper glaze layer). The result could be the crystallization of a spodumene layer thereby introducing its inversion and associated sudden expansion at 1082 C during cooling.
-Notwithstanding the previous, Li2O can even calculate inaccurately with low additions (when it is being introduced as a new material in a glaze). This is especially true when the existing glaze is not balanced (lacking in SiO2/Al2O3 or flux saturated).
-Its expansion is much lower than soda or potash, and it is used to produce special low-expansion bodies and glazes which are resistant to heat shock. When used as a partial substitute for sodium and potassium oxides, it produces glazes of lower expansion. But if it is simply added to a crazing glaze already containing significant KNaO, the crazing problem will not likely be fixed.
-Lithia gives the most intense colors in low alumina high alkali glazes.
-The alkalis can increase lead solubility.
-It can promote textural or variegated effects in the glaze surface because it is reactive and promotes devitrification in glass systems.
-Lithia can promote bubble defects in glazes if used in isolation from the other alkalis.
-In some systems, small additions of lithium will react with quartz during firing and can eliminate the alpha-beta quartz transition in the cooling cycle.
These GLFL tests and GBMF tests for melt-flow compare 6 unconventionally fluxed glazes with a traditional cone 6 moderately boron fluxed (+soda/calcia/magnesia) base (far left Plainsman G2926B). The objective is to achieve higher melt fluidity for a more brilliant surface and for more reactive response with colorant and variegator additions (with awareness of downsides of this). Classified by most active fluxes they are:
G3814 - Moderate zinc, no boron
G2938 - High-soda+lithia+strontium
G3808 - High boron+soda (Gerstley Borate based)
G3808A - 3808 chemistry sourced from frits
G3813 - Boron+zinc+lithia
G3806B - Soda+zinc+strontium+boron (mixed oxide effect)
This series of tests was done to choose a recipe, that while more fluid, will have a minimum of the problems associated with such (e.g. crazing, blistering, low run volatility, susceptibility to leaching). As a final step the recipe will be adjusted as needed. We eventually evolved the G3806B, after many iterations settled on G3806E or G3806F as best for now.
I am comparing 6 well known cone 6 fluid melt base glazes and have found some surprising things. The top row are 10 gram GBMF test balls of each melted down onto a tile to demonstrate melt fluidity and bubble populations. Second, third, fourth rows show them on porcelain, buff, brown stonewares. The first column is a typical cone 6 boron-fluxed clear. The others add strontium, lithium and zinc or super-size the boron. They have more glassy smooth surfaces, less bubbles and would should give brilliant colors and reactive visual effects. The cost? They settle, crack, dust, gel, run during firing, craze or risk leaching. Out of this work came the G3806E and G3806F.
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).
Saving energy in glass with lithium
Fluxes are the reason we can fire clay bodies and glazes in common kilns, they make glazes melt and bodies vitrify at lower temperatures.
A way of establishing guideline for each oxide in the chemistry for different ceramic glaze types. Understanding the roles of each oxide and the limits of this approach are a key to effectively using these guidelines.
In ceramics, feldspars are used in glazes and clay bodies. They vitrify stonewares and porcelains. They supply KNaO flux to glazes to help them melt.
Frits are made by melting mixes of raw materials, quenching the melt in water, grinding the pebbles into a powder. Frits have chemistries raw materials cannot.
Spodumene is a lithium sourcing feldspar, an alternative to lithium carbonate to supply Li2O to ceramic glazes. Contains up to about 8% Li2O.
A powerful melter very valuable in ceramic glazes. It is 40% Li2O and has an LOI of 60% (lost as CO2 on firing). This material in now incredibly expensive.
|Oxides||F - Fluorine|
|Glaze Color||Lithia can produce blue effects with copper.|
|Glaze Color||Lithia can produce pinks and warm blues with cobalt.|
|Glaze Variegation||Lithia contributes to mottled and flow effects when used in small amounts (-1%).|
|By Tony Hansen|
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