March 2026: We are doing major upgrades to code here, please be patient regarding any issues. If any page is not working for a period of hours, please contact us. Thank you.

Monthly Tech-Tip from Tony Hansen SignUp

Feb 14-15, 2026 - Major Server Upgrade Done

No tracking! No ads!

All Glossary

Limit Recipe

"Recipe logic" is the ability to sanity-check ceramic glaze recipes on sight, by noting that materials present and their relative percentages.

Key phrases linking here: unbalanced recipes, unbalanced recipe, balanced recipes, balanced recipe, recipe limits, limit recipes, sanity-checks, limit recipe, sanity check, recipe logic - Learn more

Details

A "limit recipe" is a sanity check. It is "recipe logic". The concept in ceramics applies to recipes of glazes, engobes, underglazes and to clay body recipes and casting slip recipes. This page deals with glazes.

There is a proliferation of pottery glaze recipes online, many of which just don’t work for functional ware. Or which impact the safety, strength and quality of your ware. Admittedly, many recipes have accompanying warnings against their use on functional ware. Or notes that they are intended for use as an over-spray to enhance the surface of an existing glaze. However, potters and hobbyists often just want to get the surface they see in a picture!

When glazes do this on drying, there is too much clay in the recipe. That is easily fixable.

There are some simple sanity checks, things to know that enable identifying obvious issues, especially potential toxicity and crazing. Remember, these limits are for functional glazes, ones whose surface will touch food and that are expected to be durable. If you want to dive deeper consider also the matter of balanced chemistry.

The concept of a limit recipe is akin to a limit formula (except it is applied on the physical material level rather than on the oxide chemistry). 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! Consider some examples:

Glazes need clay to suspend the slurry and harden them on drying. The ideal is 15-20% kaolin or ball clay. This assumes the brands typically used in glazes, e.g. EPK or No. 5 Glaze in North America. Highly plastic clays can do the job at a lower percentage; non-plastic ones require more. Other clays are also used (and clay-like materials like Gerstley Borate). Where clay is lacking, it is common to augment with bentonite, up to 5%. Where more clay is needed (e.g. to supply Al₂O₃ for a matte), some calcined or roasted clay can be used to keep the raw clay material less than 25%. If the percentage of clay is too high or the type of clay is too plastic, cracking and peeling of the dried layer will result.

A pigmented glaze runs and crystallizes like this on a test tile, question its durability and leach resistance.

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. If there is none the glaze will likely over-melt and craze. If there is 40% it won’t melt enough.

Middle and low-temperature glazes need significant boron (B₂O₃) to melt them. It comes from Gerstley borate and frits. What if a glaze has none of these? How will it melt sufficiently? Perhaps it employs zinc or lithium as melters. But these are troublesome. Are you ready for that? The Gerstley Borate (GB) deposit is exhausted as of 2022, it is no more, so there is no point putting effort into glazes containing it. It also presents serious issues in slurry usability and should be avoided anyway. There are GB substitutes that likely use Ulexite or Colemanite, and they may or may not work, but frits are superior. There are lots of pages on this site about how to source boron from a frit instead.

It is very common to see high feldspar percentages (up to 80%) in stoneware glazes. 30% is normal, 50% is not. There are very few circumstances in which excessive feldspar does not produce crazing.

Does a recipe contain “borax frit”? That is like a cake recipe containing “a grain flour”. Which one? There are hundreds of borax frits, they serve temperature ranges from cone 010 to 10, which one? Some contain 2% B₂O₃, others 30%. The other 98% or 70% could be anything.

When you see a glaze containing 50% feldspar, a "crazing red light" should go on in your head.

Does it contain a feldspar you don't have? Is the chemistry available, or is it at least known if it is a potash or soda feldspar so that you could at substitute it for another of the same type? Specific kaolins and ball clays are known to work well in glazes; consider using them instead of those listed.

Does it contain materials you have never heard of or used? Is there information there to explain their purpose? Does it contain significant bone ash? Bone ash is a phosphate, generally used for non-functional pigmented surfaces. It is not typical in glazes and not well studied or understood, especially for functional ware.

It is normal to see 1% cobalt, it is a powerful colorant. And super expensive. But if there is 5% that is crazy for functional ware, it is a leaching hazard. 3% copper oxide is normal, 10% is not. Carbonate colors (like copper, cobalt) are trouble, they gas during firing and produce blisters (the oxide forms are better). Stains are better yet.

Does it contain titanium dioxide or rutile? These are used to variegate surfaces and it is normal to see up to about 5%. In excess of this will cause heavy crystallization of the surface, inducing issues with cutlery marking and staining.

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, can you source CaO and MgO from wollastonite and talc instead?

Are you going to do multi-layering? You cannot just take any recipe found online and expect it to be suitable for multi-layering or as a first-coat dipping glaze.

Another aspect where recipe logic applies is the practice of having a dozen, or dozens, of completely different recipes in your operation. Learn from what commercial glaze manufacturers do. They use base recipes, for example, a transparent one; to that they add pigments, opacifiers and variegators. They use frits and stains; they are 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). Worry about finding ways to save money later by adjusting a working recipe.

Equipped with recipe logic, you will quickly be able to predict glazes likely to craze, leach, run, settle, crystallize, pinhole, bubble and blister. For artists, the opposite can also be true; they often seek these effects as part of an aesthetic.