Identifying the mechanism of a ceramic glaze recipe is the key to moving adjusting it, fixing it, reverse engineering it, even avoiding it!
Identifying a mechanism means you identify the reason a glaze does something specific. Especially visual. Most glaze recipes can be separated into two parts: the base and the mechanism of the color, opacity and variegation. The base is likely just a transparent glossy or translucent matte (although we can also go to a deeper level and talk about the mechanism of the matteness of a glaze, for example). Most often, a 'base glaze with variations' model is the best way to build a suite glazes or transfer them to another temperature. A base glaze that is 'understood' can be improved over time, made more and more functional and easier-to-use, less and less mysterious and prone to problems. When this base is improved, variations based on it benefit also. Imagine a base that is nice to use and apply; never cracks on drying; does not settle out; is reliable; cost effective; resistant to leaching, crazing, and cutlery marking; is gloss and temperature adjustable; and is easily opacified, colored, and variegated, etc. Would it not make good sense, where possible, to transplant the 'mechanisms' from new glaze recipes into your base rather than parachute the whole recipe (with its new materials and problems) into your operation?
Again, the 'mechanism' of a glaze is the particular material(s) or process(es) that make it do the special visual thing it does. For example, consider a glossy white: it is likely just a glossy transparent base with added zircopax or tin. If you already have a glossy glaze that has the above benefits, then why not just put the opacifier into it?
Consider the mechanism of matteness. A glaze recipe might fire matte because:
-It has a high alumina and low silica ratio and the alumina stiffens the melt so that the microsurface is rippled.
-It is not being fired high enough to melt properly.
-It is covered with a fine mesh of crystals that grow during cooling.
Knowing something about the mechanism of a glaze will give you direction if you need to make adjustments. For example, to make an under-fired matte glossier you add feldspar, to make an alumina matte glossier you add silica, to make a crystal matte glossier you cool more quickly or increase kaolin to stiffen the melt and impede crystal growth.
Consider a fairly complex mechanism: A copper red glaze. It is a low alumina, high melt fluidity glaze with a little copper and tin and fired in reduction on a low iron body. Within this system you can introduce a little boron to move the color toward purple. Knowing this will go a long way to troubleshooting and adjusting it. But moving the mechanism to another fluid base to fire at a lower temperature, for example, will be less practical; it would be better to adjust the chemistry of the source recipe by adding more flux.
Consider another benefit on knowing 'mechanisms'. In the past you may have mixed recipes from others or textbooks without regard to the mechanism at play in recipes and firing. But when you understand, you see recipes differently and make more educated judgments about whether you want them in your operation. Consider reactive glazes, glazes that really dance with variegations in color and opacity. We know that it is normal for a middle temperature glaze to have 20-30% boron frit to get a good melt (about 0.3 molar equivalents of B2O3). But if you see a glaze that contains 50-60% frit (0.6 molar), a red light should to turn on; this is really going to melt alot. And it is going to be very particular about thickness, cooling in the kiln and the contours of ware you put it on. Knowing this up front is very important to saving yourself a lot of headaches and disappointments. Like copper-reds, this type of glaze cannot really be broken into a base and a mechanism, the fluid nature of the base coupled with the colorants combine to form the visual character.
Some mechanisms live within narrow confines of firing temperature, process and applications methods. An example is a cone 10R dolomite matte. These glazes have around 0.4 molar equivalents of MgO (very high) in a fluid low silica base whose fluxes are dominated by CaO. The fired glass contains phase changes these produce the silky surface texture. Fiddling with the amount of MgO or the material which sources it is going to affect the character of the fired surface. Changing the SiO2:Al2O3 ratio will likewise have an impact. Or the source of the CaO.
Reverse engineering a glaze is a task where it is particularly important to identify mechanisms. It is not just a matter of having the chemistry of a glaze analyzed and expecting that a calculated mix of materials that supplies the same chemistry will fire with the same result. That is wishful thinking! Because so many factors can be at play. It is crystallizing on cooling? Does the color suggest a colorant or mix of colorants that requires a particular type of chemistry in the base? Or does the glaze surface suggest a particular cooling curve? Will it likely only work on a particular body type and why? How much melt fluidity does it need to have and from what flux should it get this? Does it contain entrained bubbles suggesting raw sources of calcia or magnesia were used? Does it have faults (like crazing) that suggest it contains high-soda materials? Etc. There can be so many factors that play to produce the color, surface texture, matteness, speckle, variegation, etc. For variegation, especially, it can be very hard to identify the precise mechanism. How much of the fired appearance derives from the manner of firing, the mineralogy or other physical characteristics of the materials (e.g. particle size), the method of application, etc. Identifying the exact mix of coloring oxides is not always easy, even if you were to have an analysis done (few labs know how to measure ceramic oxides, like boron, well; each oxide has its own analysis idiosyncrasies, and there can be alot of elements in a glaze). Glazes that contain unusual frits or mixtures that include special purpose frits will likely be very difficult to duplicate chemically until the right frit has been located. Also, glazes that contain Li2O or BaO from a frit will not behave the same if you supply these oxides from raw materials.
In the end, seeing the mechanisms of a glaze will involve experience and the ability you have built to separate the visual presence of the glaze into the likely recipe, material, process and chemistry factors that are likely contributors. It can be quite simple in many cases but very complicated in others.
"Mechanisms" are specifics about the glaze application or preparation process, the materials, the chemistry or firing schedule that produce a specific visual effect. This is fired at cone 10R. It is made from a buff stoneware, Plainsman H550, and has L3954N black engobe on the inside and part way down the outside. The transparent glaze on the inside gives the black a deep vibrant effect. The outside glaze is G2571A with 3.5% rutile and 10% zircopax added (the latter imparts opacity and the former produces the variegated surface). The powerful color of the black engobe wants to get through but it is only able to do so where the glaze layer is thinner (producing the varied shades of brown with differing thicknesses of glaze that occur because of the presence of the incised design).
Right: Ravenscrag GR6-A transparent base glaze. Left: It has been opacified (turned opaque) by adding 10% Zircopax. This opacification mechanism can be transplanted into almost any transparent glaze. It can also be employed in colored transparents, it will convert their coloration to a pastel shade, lightening it. Zircon works well in oxidation and reduction. Tin oxide is another opacifier, it is much more expensive and only works in oxidation firing.
You found some recipes. Their photos looked great, you bought $500 of materials to try them, but none worked! Why? Consider these recipes. Many have 50+% feldspar/Cornwall/nepheline (with little dolomite or talc to counteract their high thermal expansion, they will craze). Many are high in Gerstley Borate (it will turn the slurry into a bucket of jelly, cause crawling). Others waste high percentages of expensive tin, lithium and cobalt in crappy base recipes. Metal carbonates in some encourage blistering. Some melt too much and run onto the kiln shelf. Some contain almost no clay (they will settle like a rock in the bucket). A better way? Find, or develop, fritted, stable base transparent glossy and matte base recipes that fit your body, have good slurry properties, resist leaching and cutlery marking. Identify the mechanisms (colorants, opacifiers and variegators) in a recipe you want to try and transplant these into your own base (or mix of bases). And use stains for color (instead of metal oxides).
Iron oxide is an amazing glaze addition in reduction. It produces celadons at low percentages, then progresses to a clear amber glass by 5%, then to an opaque brown at 7%, a tenmoku by 9% and finally metallic crystalline with increasingly large crystals past 13%. These samples were cooled naturally in a large reduction kiln, the crystallization mechanism would be much heavier if it were cooled more slowly.
GR10-G Silky magnesia matte cone 10R (Ravenscrag 100, Talc 10, Tin Oxide 4). This is a good example silky matte mechanism of high MgO. The Ravenscrag:Talc mix produces a good silky matte, the added tin appears to break the effect at the edges.
The glaze on the left is called Tenmoku Cone 6 (a popular, and old, CM recipe). It is 20% calcium carbonate, 35% Custer feldspar, 15% OM4 Ball Clay and 30% silica, 10% iron oxide. If you have any experience with glaze you will note two things that a fishy here: There is no boron, lithia or zinc sourcing material. How can this melt enough at cone 6? It looks melted, but the ease of scratching it shows it is not. So, it appears that if we saturate an incompletely melted glaze with a lot of refractory brown colorant on a dark body the effect can be black. A better idea is the glaze on the right. We start with a stable, reliable base transparent, G2926B. Then we add 5% Mason 6666 black stain (stains are smelted at high temperatures, quenched and ground, they are inert and relatively safe). A bonus is we end up with a slurry that is not nearly as messy to use and does not turn into a bucket of jelly.
2, 3, 4, 5% rutile added to an 80:20 mix of Alberta Slip:Frit 3134 at cone 6. This variegating mechanism of rutile is well-known among potters. Rutile can be added to many glazes to variegate existing color and opacification. If more rutile is added the surface turns an ugly yellow in a mass of titanium crystals.
These are two cone 6 matte glazes (shown side by side in an account at Insight-live). G1214Z is high calcium and a high silica:alumina ratio (you can find more about it by googling 1214Z). It crystallizes during cooling to make the matte effect and the degree of matteness is adjustable by trimming the silica content (but notice how much it runs). The G2928C has high MgO and it produces the classic silky matte by micro-wrinkling the surface, its matteness is adjustable by trimming the calcined kaolin. CaO is a standard oxide that is in almost all glazes, 0.4 is not high for it. But you would never normally see more than 0.3 of MgO in a cone 6 glaze (if you do it will likely be unstable). The G2928C also has 5% tin, if that was not there it would be darker than the other one because Ravenscrag Slip has a little iron. This was made by recalculating the Moore's Matte recipe to use as much Ravenscrag Slip as possible yet keep the overall chemistry the same. This glaze actually has texture like a dolomite matte at cone 10R, it is great. And it has wonderful application properties. And it does not craze, on Plainsman M370 (it even survived a 300F-to-ice water IWCT test). This looks like it could be a great liner glaze.
This is the Ravenscrag slip cone 6 base (GR6-A which is 80 Ravenscrag, 20 Frit 3134) with 10% Mason 6006 stain. Notice how the color is white where it thins on contours, this is called "breaking". Thus we say that this glaze "breaks to white". The development of this color needs the right chemistry in the host glaze and it needs depth to work (on the edges the glaze is too thin so there is no color). The breaking phenomenon has many mechanisms, this is just one. Interestingly, this transparent base has more entrained micro-bubbles than a frit-based glaze, these enhance the color effect.
This is cone 6 an oxidation transparent glaze having enough flux (from a boron frit or Gerstley Borate) to make it melt very well, that is why it is running. Iron oxide has been added (around 5%) producing this transparent amber effect. Darker coloration occurs where the glaze has run thicker. These are all simple mechanisms, which, once understood, can be transplanted into other glazes. This glaze is also crazing. This commonly occurs when the flux used is high in K2O and Na2O (the highest expansion fluxing oxides). K2O and Na2O produce the brilliant gloss. They come from feldspars, nepheline syenite and are high in certain frits.
This cone 6 white opacified glaze has an addition pigment-bearing granular mineral to create speckle (e.g. illmenite, manganese granular, ironstone concretions). This speckling mechanism can be transplanted into almost any glaze. Unfortunately, the metallic particles that produce the speck are often heavy and settle quickly in the glaze slurry. This can be prevented somewhat by flocculating the slurry.
Opacity of ceramics glazes is normally achieved by adding an opacifier like tin oxide or zircon. However there are chemical profiles that can turn transparent glazes milky and make it cheaper to opacify them.
This term refers to critical thinking ability that potters and technicians can develop to recognize recipes having obvious issues and merit, simply by seeing the materials and percentages.
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