|Monthly Tech-Tip |
In ceramics, glazes are developed and mixed as recipes of made-made and natural powdered materials. Many potters mix their own, you can to. There are many advantages.
Potters in affluent places in the world have increasingly adopted commercial brushing glazes as their glazing method of choice. These have some clear advantages but also some important disadvantages (especially related to whether they fit the body, the fact that their recipe is unknown so they are not adjustable and the problem that they take so long to apply and dry slowly). In industry likewise, companies in more affluent countries have changed from mixing their own glazes (and understanding them) to buying them premixed from suppliers (and knowing nothing about their makeup).
Glaze mixing is "swimming upstream" in this river! However, potters and companies in developing countries still make their own glazes. And everywhere there is still a strong movement among potters to adjust and mix their own glazes. It is practical if you can learn about why glazes do what they do, what they need to do to work in your application and you are willing to do plenty of testing to develop your own. Surprisingly, with a little effort, you will find yourself being quite critical of commercial products that you used to use or that competitors still use, finding that many of these are actually quite poorly designed. Armed with an account at Insight-live.com to plan and record your testing work, all all the reference material it provides (like this page), you will be surprised at the results you can obtain.
The way glazes fire is primarily a product of their chemistry. Most glazes are a base transparent with additions to opacify, color, variegate and modify surface texture. The base needs to have the right thermal expansion to fit the body, stay in suspension in the slurry, apply evenly and dry quickly, not be prone to surface defects like pinholes and blisters, not leach or cutlery mark and work with a variety of additives. Other factors include optimization to cut material costs, lower firing costs and be adapted to idiosynchaeies of the body. Additions to that base (depending on percentage) usually affect some of the above properties so extra work is often needed to compensate.
The chemistry of glazes is sourced by ceramic materials. The chemistry is called a "formula". The mix of materials to produce the chemistry is called "the recipe". Recipes are akin to those used in a kitchen. They specify amounts-by-weight (that may or may not sum to 100). You simply recalculate the total and weigh out the needed amount of each powder. Typically around 2500 grams of dry material mix with 2500 grams of water produce a gallon of about 1.45 specific gravity.
Glaze mixing makes the most sense for cover and liner glazes, especially if you want to do production. Glazes for food surfaces carry responsibility and there is no substitute for knowing the recipe and being in control of the properties and fit with your clay body. Liner glazes should typically be uncolored, have low colorant content or employ non-toxic colorants. Of course for decorative, non food surfaces, exotic premixed brushing glazes and layering can make sense in low-production situations.
Glaze mixing is also about rheology, that is, producing a slurry that has good working properties. This is an area where mixing your own glazes can really pay off. It is possible to produce a glaze that goes on silky smooth and evenly and dries within seconds on bisque ware (compared to laborious multi-coat painting and long drying sessions between each coat).
Potters often fall victim to the "illicit online trafficking" of recipes that do not work. Typically these recipes produce some special effect while punishing the potter in many other ways (thus the 'illicit' designation). Not sure what I mean? You will find out! Based on the above-mentioned base-with-additions approach to glazing, it is far better to employ base transparent recipes proven to work with your body and add the colorants, opacifiers and variegators to them to get the effects you want. For reactive glazes, ones that run and crystallize, you can do the same, using a melt-fluid base that you understand and can maintain some control over. Of course, you have all of your testing and trial work recorded in your account at Insight-live.com so you learn from both your successes and failures.
These are two pallets (of three) that went on a semi-trailer load to a Plainsman Clays store in Edmonton this week. They are packed with hundreds of bags of powders used to mix glazes. More and more orders for raw ceramic materials are coming in all the time. Maybe you are using lots of bottled glazes but for a cover or a liner glaze it is better to mix your own. And cheaper! And there are lots of recipes and premixed powders here to do it. One of the big advantages is that when you dip ware into a properly mixed slurry it goes on perfectly even, does not run and dries on the bisque in seconds. No bottled glaze can do that.
Yes. In this case the entire outside and inside of the mug need an evenly applied coat of glaze. In production, it would not make sense to attempt this by painting. For these reasons: Cost, quality, convenience. The right pail has 2 gallons of G2934Y base with 10% Cerdec yellow stain: $135. Cost of jars with the same amount: Almost $300! And you have to paint them on in three coats with drying in between. The one in the pail is a true dipping glaze (unlike dipping glazes sold by glaze manufacturers that dry slowly and drip-drip-drip just like brushing ones). This one dries immediately after dipping in a perfectly even layer (if mixed according to our instructions). And a bonus: This pail can be converted to brushing or base-layering versions using CMC gum.
All of the equipment has been washed in preparation for a porcelain run. Original container bags are broken in the dust-hood unit on the right and augered and elevated into the rotating blender/mixer. It feeds a vibrating screen (not visible) that removes paper and other contaminants. For wet clay bodies the screen feeds hoppers on the other side of the wall, they in turn feed the pugmill. For dry bodies and glazes the powder goes to one of the hoppers and that feeds a bagging unit. This type of equipment can handle 1200 lb batches (doing one every five minutes for some products, longer for others).
Hobbyists and increasing numbers of potters use commercial paint-on glazes. It's convenient, there are lots of visual effects. There are issues compared to dipping glaze. You can make your own.
A database website where potters and ceramic technician account holders enter their recipes, materials, pictures, test procedures, firing schedules, etc.
Glaze chemistry is the study of how the oxide chemistry of glazes relates to the way they fire. It accounts for color, surface, hardness, texturem, melting temperature, thermal expansion, etc.
Where do I start in understanding glazes?
Break your addiction to online recipes that don't work. Get control. Learn why glazes fire as they do. Why each material is used. Some chemistry. How to create perfect dipping and drying properties. Be empowered. Adjust recipes with issues rather than sta
Concentrate on One Good Glaze
It is better to understand and have control of one good base glaze than be at the mercy of dozens of imported recipes that do not work. There is a lot more to being a good glaze than fired appearance.