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Written in the 1990's

Waste Glazes may not be appropriate for food surfaces. Don't waste good pots with bad glazes.

Ask the Doctor:Preventative Medicine

"Waste Not"

Dear Doctor, I goofed mixing a bucket of a glaze wrong. I apparently switched recipes mid-stride. Can you help me find a use for this runny green glop.


Oops in Michigan,

Dear Doctor, Our glaze area sink does not have a trap, so we have been washing brushes, buckets and other glazing tools in trash cans filled with water. The second can is almost filled. I would hate to throw away this material. What should I do?


To the Rim in Arizona,

Dear Oops and Rim,

I applaud your desire to conserve resources, protect the environment, save money and avoid violating the first principle (never throw anything away ). My first experience with recycled glaze ingredients was when my job was to empty a fifteen gallon trap built into the floor at the University of Michigan in 5735 (1975). This material without any alterations was an acceptable cone ten semi-matte black. If I remember correctly we called it Scrap Black. It was not very popular, it might still be there..

My next experience was while my wife was teaching at University of Missouri, Columbia. We had inherited a good deal of glaze remnants with little documentation and a five gallon trap full of glaze material. We mixed it all together and developed a few glazes with these materials. Named "Moonlight over Montana" and other colorful names these firing sensitive glazes with complexly colored and surfaces were studio favorites until they ran out.

In the Island University studio I (used to) segregate the clay washing sink from the glaze sink. This limits the variation in the scrap and insures that the glaze scrap will at least come close to melting. It also limits the amount of red clay in the scrap and keeps the scrap glaze from always being a tenmoku. It is a good idea when possible to keep glaze scrap separated by temperature. Since we fire most everything to cone 10 this is not a problem for us.

When altering the glaze scrap I try to use only inexpensive ingredients, and only those that are very O.K. to dispose of in landfills. I mix and sieve the scrap and test it in our normal firing cycle. If the scrap has chunks in it that don't want to mix in I throw the chunks away.

Runny glazes that craze get additions of clay; sometimes a ball clay, sometimes kaolin, occasionally a red clay. I use additions of about 2 pounds to five gallons. I don't bother weighing it. Glazes that craze but are not runny get silica straight up or talc and silica. I use the talc when I am short on matt glazes in the studio. Scrap that doesn't seem quite melted usually gets whiting or dolomite, or occasionally plastic vitrox (we are long on this material).

I favor red iron oxide, and rutile as colorants to add to scrap glazes. For five gallons I use about 250 grams (1/4 pound). When the scrap glazes start to build up however, some cobalt carbonate will increase usage. If the glaze is not already too dark, cobalt carbonate 100 grams should make a big difference in five gallons of glaze. With cobalt's cost, it better make a difference. I usually  prefer to use the big dollar colorants in tested glazes.

A common fault of these scrap materials is over-floculation. This is often caused by an excess of soluble ingredients in the scrap. Over-floculation can cause crawling. To wash out the soluble salts, split the materials into a few 1/4 full buckets. Add water until full, stir and let the scrap settle out. Throw the water from the top of the buckets away. If the problem is severe, wash the materials twice.

Scrap materials build up fast and fixing five gallon buckets has become too much work. It seems that with good reclamation about 15 gallons of scrap glaze appear semester down here at the Island University. I am beginning this semester to fix the scrap glaze in 15 gallon quantities. We get the buckets from community swimming pools. They get pool chemicals in them. The buckets come with nice screw down plastic lids. The buckets contained very hazardous stuff, keep it out of your eyes and don't breath it. Read the labels.

When a glaze looks especially nice, or I do a good demonstration pot with it, it gets used quickly. The students know there will be no more. When I am demonstrating glazing or onglaze decorating I often use an unpopular scrap glaze, and wash the pot off afterward, so I can use the bisqueware again.

Gail Busch, my wife, always washes her sigillata brushes in the same cup of distilled water. Her scrap sig usually comes out a gray blue green.. Scrap Raku glazes usually need no alteration,. and have been our most popular surfaces. If they don't look good try adding copper carbonate.

The key to recycling is to spend you efforts and materials where they will do the most good. If a batch of scrap just won't work, throw it away, and concentrate your efforts more productively on a batch that looks promising. If a batch looks like it will require a large alteration to make it work, throw it away. Teach your students to use a spatula to recover glazes from pouring containers and ladles. Back to the studio.....,

2018 note

I now generally separate out one quart from 4 gallons of scrap and test alterations on the small quantity. Occasionally I make a low grade iron oxide from roasted rust I use that sometimes. I also have some odd lots of colorants from donations from local studios. I use those sometimes. I am now almost routinely using my soluble removing procedure from scraps. If there is much soda ash in the scrap it vastly reduces its usability.

What to do with dark glazes is difficult. Sometimes you can move them to black with more iron or cobalt. Some suggest adding rutile. Watering the color down takes a lot of material and seems like throwing good material after bad.


Waste Glazes may not be appropriate for food surfaces.

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Page last modified on February 11, 2020, at 08:27 PM