Glazes

Emmanuel Cooper – Glaze test tiles

Understanding glaze, its formulation, application and its final quality, are essentials to creating a successful final piece. The sheer breadth of quality and decoration that can be achieved through glazing is arresting. Earthenware or Stoneware? Oxidation, Reduction, Raku, Salt? Layers? Just endless possibilities . However, this area of ceramic practice is potentially the most volatile and impactful on your work.

Glazing is often considered an art form in itself. It can take years to become proficient, but the endeavour is rewarding  in the quality of the finished objects.

In its simplest form a glaze is a thin coating of glass that fuses to the clay body during the firing process. Raw materials are weighed to a recipe and mixed together in water before being applied.

The act of glazing is important on a number of levels. In functional pieces, glaze is used primarily to finish the fired ceramic surface, making it smooth, nonporous and hygienic. In a more artistic context, glazing is a mean of expression in its own right.

Glazes can be bought from pottery suppliers either as a ready mixed liquid or a powder to which you add water. However, here at our great facilities, we have an array of chemicals where we can basically mix any glaze and if we can’t find a specific compound there is always an alternative one. Whatever the medium you still need to observe the conventions of glazing and health and safety requirements.

Picture1

A glaze is made up of three constituents: silica or silicon dioxide which is the main ingredient to form the glassy surface; a flux such as zin oxide or calcium oxide to control the melting point of the glaze; and aluminia or aluminium dioxide to stabilise the glaze and bind it to the clay body. Many materials may be mixed together, but they must still make up these three separate parts. Decorative qualities or colours are introduced to the basic glaze through the use of pigment or metallic oxides such as cobalt oxide.

Glaze presentation

We were taken on an induction on glaze mixing with Duncan, going through all the process and health and safety precautions. I had a looked at a few books from the library and looked at some recipes to work on. The actual process of mixing a glaze is quite simple, but needs to be precise. Simply, measure all the powdered chemicals separately add them all together mixing 1:1 them with water; then sieve the mixture through a 100 hole mesh ensuring you got the last bit of it, only this way we can be sure all the chemicals have been mixed uniformly together. And finally, add a label due to health and safety and simply for inditification prepuces. Its finally ready for testing.

On my first go I looked at 3 glazes,  a earthenware glaze, an opacifier glaze and a raku (yes, because we have an induction soon – can’t wait)

Really happy how this lead glaze with an addition of iron oxide came out. Really like how it affected the rim of this pinch bowl.

Lead Glaze with addition of Iron Oxide – 1080ºC

  • Lead Bi-Silicate                       65
  • Whiting                                      10
  • Potash Feldspar                       15
  • China Clay                                 10
  • Red Iron Oxide                         6%

IMG_4365.JPG

Just seeing this picture makes my heart slowly sinks into the back of my soul. Me, being me, just brushed this opacifier glaze directly onto this bisque teapot; and this just shows the importance of test glazing and looking into application methods (dipping, spraying). So from now on, TEST GLAZE. Not only I risk damaging my work but I also risk damaging other peoples work and also kiln furniture as glaze can flow onto it.

Opacifier Lead Glaze with addition of Zirconium 1080ºC

  • Lead Bi-Silicate                       65
  • Whiting                                      10
  • Potash Feldspar                       15
  • China Clay                                 10
  • Copper                                       1.5%
  • Zirconium                                 5%

Here are some more examples of what not to do. I applied this white earthenware glaze by dipping  my pieces but I believe it was added too thickly therefore the glaze crawled.

Something really important in general in ceramics, specially when it comes to glazing is keeping a detailed record of your glazing experiments which will help you to reproduce effective glaze recipes in the future and well as you won’t forget them. You should record information such as firing range, thickness of glaze application and the outcome either on your journal or an app such as Ceramics Archives.

Even though my glazes didn’t go as I expected I just need to persist, by looking at glaze applications and test glazing glazes. It’s something that can be frustrating but I just can’t give up on my first try and that I actually am interested on.

 

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Published by

tonidej

BA (Hons) 2nd Year Ceramics student at Cardiff Metropolitan University. Love experimenting with material and techniques and work on a range of medium.

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