Glaze testing

Updated: Feb 22, 2020

Since I started ceramics as a hobby I've been using off the shelf glazes bought from a number of suppliers. There are all sorts of different types, brands, colours, textures to choose from and it can be a reliable way of creating gorgeous glazes. However my partner bought be a book for Christmas called The Complete Guide to Mid Range Glazes by John Britt and I've been completely obsessed with mixing my own since then. There are so many ways to decorate ceramics- scratching into the surface, painting with brushes, drawing or dotting on with slip (liquid clay) and so on. But I feel myself being drawn to the 'simplicity' of the effect a glaze can have when it interacts with the surface of the clay when fired. Glossy candy like swirls or rich and vibrant colour that looks like it's going to drip off a mug, or the depths achieved with crackle (or crazed) glazes that reveal a little of the fired clay beneath round edges and ridges. I am fascinated by the chemistry behind this and what makes glazes work and not work- which is often down to creative interpretation or the intended use of the finished item. A crackle glaze is often thought of as faulty in foodware while deemed beautiful on decorative pieces.

Hankering to start mixing my own glazes, but daunted, I followed recommendations by experienced potters on instagram and signed up to 'Cone 6 Glazes' by This comprehensive course takes you through the complex work of glaze chemistry in a clear and engaging way, and I feel I have a vastly increased understanding of why glazes behave in certain ways and what ingredients I could adapt to manipulate that behaviour to suit my needs. It's early days- there is a huge about of information to digest, actively test out and try and implement. However the course gave me the confidence to find a starting point and I have now mixed my first transparent, glossy 'base' recipe successfully- and tested added colourants to that.

My base recipe is one suggested by Matt Katz the course author/owner and I added copper oxide, cobalt oxide, manganese carbonate, zirconium, black iron oxide, tin and chromium. I used two clay bodies I like to throw- a buff stoneware and a white KGM stoneware. I then did two tests of each colourate on it's own- one the minimum amount recommended and one the maximum. So for Copper Oxide there were four test tiles- one buff 1%, one buff 3%, one white 1% and one white 3%. I then mixed some colorants together based either on recipes amounts I'd found or on guesstimates to try and create turquoise, raspberry and rich brown. After that I then dipped layers colours on colours- for example, one 3 second dip in cobalt then a 2 second dip in manganese.

These were then fired to cone 6 oxidation.

I'd had a sneaking suspicion that it wouldn't work. However much to my delight and astonishment it ruddy worked! I sat and examined each test tile, made note of the texture, shine, colour, movement and general aesthetic. I've never felt so studious in my life! I particularly liked copper- on it's own a gorgeous fresh green at 1% on white stoneware. While mixed with chrome it gave a dark, almost black green with metallic onyx like finish. I didn't successfully make raspberry or turquoise but I really enjoyed the blushing pale pink instead. I also accidently made a rich Indigo colour and was delighted, my daughter is called Indigo so this will definitely be part of my glaze catalogue!

I mixed and dipped a second test batch. As well as test tiles, I used some broken/ discarded bowls to also layer up some glazes and these came out with stunning success. I actually shed a little tear I was so overjoyed and surprised! In these bowls, I found turquoise with cobalt layering over manganese. And Zirconium layering over this came a vivid purple!

However raspberry remains a elusive. Further research has suggested a tweak to the base recipe so I'm now waiting on an order of fresh ingredients to arrive and I'll test again. I desperately want to add raspberry to my initial collection- 'Raspberry' was the nickname we called our daughter when still in my belly and therefore feels another important colour to explore. Our energy ball of a dog Conker is well, called conker so I rather fancy also finding a rich nutty brown - perhaps an Autumnal colour.

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