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Short history of my experiences on engobes

Definition

An engobe is a liquid clay (slurry) colored by metal sources (mainly oxides) whose aim is to cover the raw clay to give it a different color or make a scene.

Engobe can be applied by brushing , dipping or with a bulb. It is preferable to use the same clay as support to prevent chipping. The color of the glaze comes out from the transparent cover (glaze)

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My experience :

I make pottery as an amateur in an association for several years, only earthenware. There are about 4 years I bought the recommended components for producing parts by casting.

I added to my order a selection of oxides without knowing what to do with them. May be make engobes or color the loam?

Then a friend gave me a dozen other metal sources. I thought it would be nice to have some different colors. I made first 4 samples with cobalt, chrome, iron and antimony. These initial tests have yielded only cool colors and not very good; more, antimony was never yellow. So I performed some research in books and on Internet to find out how to correctly use these products. Then after asking many questions on newsgroups. I concluded that it was in mixing at least two oxides that provides interesting nuances. But I have not found a reliable guide offering more complex recipes. So I decided to do my own research. The original idea was to make preparations with a larger quantity of oxides, taking account for the tinting power, make mixtures in pairs and look for interesting combinations. This website describes what I did, and how.

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Preparation of samples with a single source metal:

I took a loaf of dry and well white clay (the clay sandstone KPC1000) than I crushed in a cloth with a rolling pin. Seeing that it was too long and laborious I made a jar mill with an engine machine according to plans of Jacques Datcharry.

I was able to grind enough clay to make my samples. Then I passed through the sieve 80.6 kg of earthen matter ; it is a long grinding and sifting. To each jar I added oxide and water and spent it all again through a sieve of 80. I used a kitchen electronic balance of 3kg maximum range and accurate to the gram. I put it in 28 bottles of 200ml drinking yoghurt as containers. To get engobes of the right consistency, I put in each bottle 250 gr of ground matter and 100ml of water with in addition 3 gr of sodium silicate per liter of water and 1 g of sodium carbonate. (carbonate and silicate of soda must be dissolved in hot water).

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List sources of metal:

  • 13 oxides: cobalt oxide, chromium oxide, iron oxide red, Black copper oxide, tin oxide, nickel oxide, Iron Oxide Black, dioxide manganese , dioxide titanium ,
  • Antimony oxide, zinc oxide, vanadium pentoxide, titanium oxide.
  • 3 Carbonate: Carbonate Cobalt, Manganese Carbonate, Copper Carbonate
  • Silicate: Silicate zircon,
  • Nitrate: Bismuth nitrate Sub
  • 2 Chromates: iron chromate, potassium dichromate
  • Yellow Ocher
  • 3 dyes in Solargil, Yellow P15, P34 Orange, turquoise P227

Preparation of iron oxide black

I had fun making my black iron oxide. By placing steel wool in a plastic bottle with water. This aerated by air bubbles generated by an aquarium bubbler gave me after a few weeks 300gr iron oxide, which I washed and filtered through a sieve of 80.

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Classifying :

My preparations are numbered, labelled; and recorded on an Excel spreadsheet.

I have added a classification code from A to A1 to sort the samples.

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Making :

The samples are placed on rectangles of 30mm by 50mm by 10mm thick of the same clay with a brush.

The first firing takes place in an electric oven then in an oxidizing atmosphere (not open during cooking). The samples are mixed (glazes and biscuits) because there is not enough production to fill the oven. There is an initial rise to 600 ° for 480mn then to 1020 ° with a plateau of 20 minutes.

I filed a blanket of C315 Ceradel then passed to the second firing

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Sample with several metal sources :

On a table (table of couples) I put on the abscissa and ordinate the samples with each source and combinations to try. I deleted the table in all the combinations involving presentations of the same metal (oxide and carbonate, for instance) and cases without interest(s). Then I realized mixtures in camera film or Doliprane case-bottles of 20m in capacity. I used a 10ml syringe. I put in each one two one source samples in equal amounts. Each sample is numbered in order of achievement and dated. Initially I just chose at random from the table but after a few tries, I treated the samples in roughly the order of the columns.

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Workbook :

Finally I put all these samples in a workbook :

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