|Monthly Tech-Tip |
Material substitutions in ceramic glaze and body recipes must consider their chemistry, mineralogy and physical properties
Material substitution is a constant and ongoing part of any ceramic enterprise that is using clay and/or glaze recipes. Often lack of availability, quality issues and price are the motivating factors. In addition, when recipes need to be used in another locality where the same raw material brands or even types are unavailable, substitutions must be done.
In ceramics, glazes fire the way they do primarily because of their chemistry. The most basic substitution is to simply replace one material for another which has a chemistry similar enough that the fired properties will not be adversely affected. This is often possible with different brand names of the same mineral or refined material (e.g. Calcium carbonate, zinc oxide). The more complex the chemistry of the raw material, the more likely there is to be issues with changes to another (e.g. feldspar). In some cases substitution recipes are recommended where a mix of two or more other materials is said to be equivalent. Another problem can occur where, although the chemistry of the substitute is very similar, it's physical properties or particle size are different enough to affect the working properties of the glaze (e.g. a kaolin) or even it's fired appearance (a metal oxide).
A given chemistry can be supplied by many different mixes of refined and raw material powders, each of these having advantages and disadvantages regarding price, ease of use in production, toxicity, etc. It is common to use glaze chemistry to calculate how to juggle a recipe to substitute one material for another of slightly or even very different chemistry while maintaining the chemistry of the glaze as a whole. The calculation is more complex where the substitute is bringing along other oxides not in the material being substituted (or fewer) or the original or substitute has a very different weight loss on firing (LOI). Frits especially can have complex chemistries and obviously it is more complicated if they are involved in the substitution calculation.
When clay bodies and porcelains require material substitution, the issue is physical properties (which are often not directly related to chemistry). Thus testing must be done to see how maturity, drying properties, plastic and/or forming behavior, texture, firing color and character, thermal expansion and other properties are affected. A series of tests usually must be done to alter the recipe to accommodate the new material while maintaining the needed properties.
G2934B is a popular recipe and there has been alarm recently because of the difficulty in getting the Ferro frit. This motivated us to get a supply of the Fusion equivalent, F-19. We have done much testing in glazes and with melt fluidity tests like this and so far it is looking good.
Fusion Frit F-19 is claimed to be a substitute for Ferro Frit 3124. The glaze was applied on Plainsman L210. Not as thick as would be customary on Majolica ware. Left is with frit 3124, right is with the Fusion product.
After comparing the chemistries of an original feldspar and a tentative substitute, these melt flow tests are an excellent way to confirm physical similarity also. These were done at cone 6 (2200F). Each feldspar is mixed with 15% Ferro Frit 3195. Some things to note: Nepheline Syenite is the champion melter. Mahavir is very similar to G200. Kingman and Custer are very similar. Our Minspar substitute is very similar to Minspar itself.
Why do this? We did not have it in stock and customers needed to mix recipes. When the chemistries of the two feldspars are very similar substitution is often not a problem, especially when a recipe only calls for 5 or 10%. However, when a recipe calls for a significant percentage the situation becomes much trickier (in our cone 6 test recipe, "Perfect Clear", 40% Minspar is needed). Feldspars are almost a glaze in themselves, just needing silica and alumina to shift their chemistry toward 'glazedom'. In this project I calculated a mix of materials, in my Insight-live.com account, that sources the same chemistry as Minspar. I made a cone 6 GLFL test comparing the Minspar and Minspar substitute (left) and comparing the Perfect Clear glaze with each feldspar (right). As you can see, the similarity in melt flow is stunning! This is a real demonstration of just how practical and valuable glaze chemistry calculation can be.
This is a GLFL test comparing the melt flow of the three materials at 1800F. Frit 3124 is barely out of the starting gate and the other two have crossed the finish line! With frits chemistry is a big deal, they are all about supplying oxides to the melt. Frit 3134 is low-alumina/high-boron, 3124 is medium-alumina/low-boron and 3195 is medium-alumina/high-boron. Boron is the melter. Alumina thickens the melt and hardens the glass. Just from this it appears that Frit 3195 is a better starting point for calculations to replace frit 3134.
Can't get frit 3134 in early 2021? You are not alone. Don't listen to people who say you can just replace frit 3134 with 3124 in glaze recipes. That is wrong. Frit 3124 has five times the amount of Al2O3 (the second most important oxide in glazes) and half the amount of boron (the main melter). The glaze chemistry approach is much better, and easier than you think. To be able to do it you need two other Ferro frits, 3110 (or Fusion F-75) and 3195 (Fusion F-2). As it turns out, Frit 3195 is more important than is 3124! A key goal in the way I do this was to end up with at least 15% kaolin (to suspend the slurry). I have chosen three types of recipes to demonstrate. Dealing with each requires a unique approach. Two of the calculations produce improved slurry properties and one a recipe of significantly lower cost. I made a video demonstrating substituting Frit 3134, see the link below. If you have a recipe that needs this, get an insight-live.com account, enter it there and I can help you do the calculation.
These are fired bars of Laguna SG758 Barnard Slip substitute going from cone 04 (bottom) to cone 6 (top). It is melting at cone 6. The bars are expanded above cone 6 and becoming quite porous. The drying shrinkage is around 7%, it is quite plastic.
Melt fluidity test showing Perkins Studio clear recipe original (left) and a reformulated version that sources the boron from Ferro Frit 3134 instead of Gerstley Borate (right). The later is less amber in color (indicating less iron) and it melts to very close to the same degree.
These flow tests demonstrate how similar the substitute recipe (left) is to the real material (right). 20% Frit 3134 has been added to each to enable better melting at cone 5 (they do not flow even at cone 11 without the frit). Links below provide the recipe for the substitute and outline the method of how it was derived using Digitalfire Insight software. This substitute is chemically equivalent to what we feel is the best average for the chemistry of Cornwall Stone.
Quartz particles have a high melting point, they must enter the glaze melt by being dissolved by it (usually the last particles to do so). Obviously the silica should be as fine as possible to increase its surface area to be more readily dissolved. The more that dissolves the closer the physical properties of the fired glaze will be its theoretical (e.g. degree of melting, thermal expansion, transparency, durability). This brand of silica, #90 (likely 45 microns) classifies as 200 mesh even though 2.8% remains on the 200 mesh screen. Not surprisingly, their #45 grade retains 1.9% on the 325 mesh screen. However, the most significant aspect is how much of the #90 is on the 325 and 270 mesh screens: 26%. The #45 grade only has 2.6! This is a huge difference and shows the value of using the finer material. It would take a typical ball mill hours to make this difference.
Dolomite is a key material for glazes, especially mattes. When you are forced to adopt a new brand it needs to be tested. Here, three tests were done to compare the old long-time-use material (IMASCO Sirdar) with a new one (LHoist Dolowhite). The first flow test is a very high dolomite cone 6 recipe formulated for this purpose; the new material runs a little more. The second is G2934 cone 6 MgO matte with 5% black stain; the new material runs a little less here. The third test is the high dolomite glaze on a dark burning clay to see the translucency and compare the surface character. They are very close. It looks like it is going to be OK. Does your supplier test new materials when they are forced to switch suppliers?
This is a melt fluidity test comparing two different tin oxides in a cone 6 transparent glaze (Perkins Clear 2). The length, character and color of the flow provide an excellent indication of how similar they are.
This GLFL test compares the melt flow, at cone 6, of two glaze recipes containing the calcium carbonates (of which they make up 27%). Notice the amount of bubbles (due to the high LOI, or loss on ignition). The 3HX is flowing a little more. This could be because of a difference their proportions of dolomite and calcium carbonate minerals, the individual mineral purities or the particle size (or all three). Whatever the case, 3HX will make a glaze flow a slightly better.
The NZK body, Polar Ice, is on the left. The Grolleg one, L3778D, is on the right. They are not the same recipe, the L3778D has been adjusted to compensate for the feldspar contained in the kaolin. Clearly, the NZK has better translucency. And it fires whiter.
Desktop Insight 1C - Substitute Wollastonite for Whiting in Glazes
Compare calcium carbonate (whiting) with other sources of CaO (dolomite, wollastonite, frit), learn to understand the chemistry differences between materials and then substitute wollastonite for whiting in a specific recipe.
Substituting Materials by Weight: Why it does not work!
Wollastonite is 50:50 CaO:SiO2. So why not just substitute 40 wollastonite for 20 calcium carbonate and 20 silica? The answer will help you see reason why we make such a big deal of glaze chemistry.
Subsitute Gerstley Borate in Floating Blue Using Desktop Insight
Use Desktop Insight to explore ways of calculating substitutes for Gerstley Borate in the popular Floating Blue cone 6 glaze recipe while maintaining or improving the other raw and fired properties of the glaze.
Creating Rules for Calcium Carbonate - Wollastonite Substitution
How to use Digitalfire Insight software to determine how much wollastonite to add and silica to remove to substitute for calcium carbonate in the glaze. Create substitution rules.
Substituting Nepheline Syenite for Soda Feldspar
Learn to substitute Nepheline Syenite for Soda Feldspar (and vice versa) using the KNaO concept in Insight. You will see the benefit of in-recipe substitution calculation rather than making general substitution rules.