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.
Because of the proliferation of glaze recipes online, potters and technicians need to develop critical thinking skills to be able identify obvious issues with new recipes presented to them. And testing skills to learn the less obvious ones. And material knowledge to recognize the mechanism of the fired appearance. And chemistry skills to evaluate that mechanism on the oxide level. And skills to fix problems, substitute materials, adjust surface/temperature or just transplant the mechanism into a base you already have working in your circumstances. The knowledge needed is here on this site, and its exciting and easier to learn than you might think.
The concept of a "limit recipe" is that we expect the percentages of material types to fall within certain ranges for typical glossy and matte functional or service glazes. If they are not red flags should pop up. Here are some examples:
Glazes need clay to suspend the slurry (and supply Al2O3 to the chemistry). We expect 10-30% in most. If a glaze has 5% clay, something is wrong. Perhaps it has 40%, that might be required to supply the chemistry needed for a matte but it will shrink and crack on drying.
SiO2 is the building block of all glazes. Most combinations of materials do not supply enough so most recipes contain silica. 10-30% is normal. What is there is none? Or 40%. Why?
Middle and low temperature glazes need significant boron (B2O3) to melt them. It comes from Gerstley borate and frits. What if a glaze has not of these? Perhaps it employs zinc or lithium as melters. But these are troublesome, are you ready for that?
Does it contain a feldspar you don't have? Is the chemistry available or is it at least know if it is a potash for soda feldspar that you could substitute. Does it contain a kaolin and ball clay that you do not have? Likely you can substitute one you do have. But being willing to do the testing is important.
Does it contain materials you have never heard of? Walk away. Or Cornwall Stone or Nepheline Syenite, these are similar to feldspar, you’ll need to calculate. Does it specify a substitute of these?
It is normal to see 1% cobalt, it is a powerful blue color. And super expensive. If there is 5% that is crazy. 3% copper oxide is normal, 20% is insane in a functional glaze. Carbonate colors (like copper, cobalt) are trouble, they gas during firing and produce blisters, the oxide forms are better. Stains are better yet.
Barium carbonate at 5% might be OK, but 20% is going to produce a toxic glaze. Lithium carbonate likewise.
High percentages of calcium carbonate or dolomite? They gas like mad, better to source CaO and MgO from wollastonite and talc.
Are you going to do multi-layering? Then you need to know how to turn the recipe you find into a first-coat dipping or brushing glaze.
Don't waste time. It is crazy to have a dozen completely different recipes in your operation. Learn from what commercial glaze manufacturers do. They create a base transparent and add colors, opacifiers and variegators to it to create an entire line of glazes. They use frits and stains, they are so much less troublesome and safer than raw fluxing materials (like lithium carbonate, calcium carbonate, zinc oxide, strontium carbonate, barium carbonate) and raw metallic carbonates and oxides (like copper, cobalt, chrome, iron). Don't try to save money, you will end up wasting time and reject ware.
Predict glazes likely to craze, leach, run, settle, crystallize, pinhole, blister.
This is recommended in the booklet "15 Tried and True Cone 6 Glaze Recipes". This melt flow tester compares it with a typical cone 6 glossy, G2926B. This recipe is 90% Frit 3110 and 10% kaolin and their booklet recommends adding stains to it. But anyone knowing a little about this frit knows it would run off this flow tester even before bisque temperatures. It is crazy to recommend this. Even as a crackle. For cone 6 it needs to be diluted much more, not just with kaolin but also silica. I knew this would run but I underestimated its melt fluidity. I put a large tile below the tester to catch overrun, yet the melt ran off that and a big three-cm-wide blob melted through the kiln wash and so far into my zircon shelf I cannot chip it off! I cannot imagine how many people have tried this on vertical surfaces and had the same thing happen. The lesson: Use common sense when looking at recipes, then you don't even need waste time testing them. Even if their authors did not!
Fired at cone 6. The samples on the bottom tiles are from ten-gram balls that have melted down. These glazes have the same chemistry, but the one of the left sources its B2O3 from Gerstley Borate (which has a high LOI). The one on the right gets it from a frit. Because the fritted version has less gases of decomposition to expel, the glass is much smoother. Curiously, the fritted version is flowing less and the red color has been lost. Why? This could be because the Al2O3, which stabilizes glazes against excessive fluidity, is being dissolved into the melt better and more available for glass building.
There are thousands of ceramic glaze recipes floating around the internet. People dream of finding that perfect one, but they often only think about the visual appearance, not of the usability, function, safety, cost or materials. That resistance to understanding your materials and glazes and learning to take control is what we personify as the dragon. Using the resources on this site you could be fixing, adjusting, testing, formulating your own glaze recipes. Start with your own account at insight-live.com.
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).
They might look great on a fancy website, but what are the chances they will actually work in your circumstances? Very low. After trying many glazes you may think you have found one that works. But does it really? Or is it erratic and unreliable? Difficult to use. Does it leach or craze or shiver or pinhole or blister? Or give you other problems? Be critical and cautious about recipes you find.
This is an example of a recipe being trafficked online that raises red flags just looking at it. The first red flag: There is no silica! That means this is a low fire glaze masquerading as middle temperature, so it is going to run during firing (run a lot). It will also mean poor durability. There is a ton of feldspar, that means a high level of sodium. Without low-expansion MgO to counterbalance it's high thermal expansion the glaze is likely going to craze badly. The mechanism of the crystallization is titanium over-supply, this has triple the maximum I would ever put in a glaze. The crystallization happens during cooling in the kiln (producing the visual effect being sought). But the the surface produced will cutlery mark and stain, probably very badly. Given the unbalanced chemistry this has, any colorant added will likely be leachable! I tested it and all my fears were realized. My slow-cool firing made the surface so dry it was very unpleasant to touch. Maybe this needs fast cooling. But who knows, there are no notes. This does not appear to belong on any functional ware, inside or outside. Someone noted that people use this to produce layering effects (see links). That begs documentation on how that wold work. Without gum would it lift and crawl as layers are added over it. Would you have to overlay every square inch? Would it still craze? All the how-to information needed to make it work are more important that the recipe itself.
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 traditional ceramics and pottery dipping glazes can be of two main types: For single layer and for application of other layers overtop. Understanding the difference is important.
Hobbyists and increasing numbers of potters use commercial paint-on glazes. It's convenient, there are lots of visual effects. But there are also issues compared to making your own.
Ceramic stains are manufactured powders. They are used as an alternative to employing metal oxide powders and have many advantages.
Loss on Ignition is a number that appears on the data sheets of ceramic materials. It refers to the amount of weight the material loses as it decomposes to release water vapor and various gases during firing.
Frits are used in ceramic glazes for a wide range of reasons. They are man-made materials of controlled chemistry with many advantages or raw materials.
Stop! Think! Do not get addicted to the trafficking in online glaze recipes. Learn how they work. Understand them. Then make your own or adjust/adapt what you find online.
There is an increasing awareness of the food safety of glazes among potters. Be skeptical of claims of food safety from potters who cannot explain or demonstrate why.
Identifying the mechanism of a ceramic glaze recipe is the key to moving adjusting it, fixing it, reverse engineering it, even avoiding it!
Trafficking in Glaze Recipes
The trade is glaze recipes has spawned generations of potters going up blind alleys trying recipes that don't work and living with ones that are much more trouble than they are worth. It is time to leave this behind and take control.