Glazing Magic!

While as a ceramic sculptor I’m all about the building and form, I have to admit that often the magic happens with the glazing. Opening a kiln after a glaze firing and seeing that your initially plain shape has taken on the colours, textures and finish of glaze can honestly seem like magic. It really makes a piece look finished and ready to be used or displayed.

For some glazing is like a magic art full of mystery and intrigue, while for others it’s a heavy science full of precision and detail. For me I feel the truth is somewhere in the middle. You need the mystery and magic to explore and be fearless while you need the precision and detail to record and be able to repeat your successes.

Glazing magic
Just a few of the glazes at my studio for students to choose from

I use mainly commercial brush-on glazes at the studio and I love them. They mean I can offer my students a range of over 100 glaze options to choose from when they’re looking to decorate their work, something I would find difficult to do with dipping glazes or if I made my own glazes due to space and time restrictions. I buy a variety of brands from a range of online suppliers and find each has particular qualities, strengths and weaknesses that need to be considered when choosing for a piece but can equally be used to advantage. The glazes can also be layered to give even more colours and effects. I’ve layered different brands with no problems (so far!) but, as is often the case with glazing, I recommend testing before trying this on any precious pieces, regardless of whether you have seen online that it made the most amazing effect for someone else.

When choosing a glaze the most important thing you need to know is actually the firing temperature of your clay! Glazes need to fit your clay body or you are likely to get problems like crawling (where you see patches of clay through the glaze as if it’s pulled away), shivering (where small pieces of glaze shiver off the surface of the clay) or crazing (where you see hairline cracks in the glaze that aren’t supposed to be there).

You need to be sure that both your clay and your glaze are happy being fired to the same temperature. Both clays and glazes usually have a firing range. For example, I work in stoneware so many of my glazes have a mid-fire range of 1200-1240C and my clay has a range that can be fired up to 1280C. I fire to 1200C most often and both the clay and glaze are good at that temperature and so make a good fit.

Sometimes glazes will mention cones rather than, or as well as, temperature. These cones refer to Orton or Pyrometric Cones which are small cones that are made of refractory material that can be placed in a kiln to give an indication of the temperature that has been reached during a firing. Each cone is made to melt and bend at a certain temperature and typically a set of three is placed in the kiln, one lower than the desired temperature, one for the desired temperature and one above. The cones can be used to as a control in the old kiln sitter controllers or as a test to be sure thermocouples are reading correctly or a kiln is firing evenly throughout. You can tell the temperature the kiln has fired to based on how each cone has bent and melted. The cones are useful as they they don’t just measure temperature but also heat work (the impact of time as well as temperature on the clay).

If you see a cone range indicated on a glaze rather than a temperature range you just need to check out a pyrometric cone chart to see what temperature each cone refers to. For example I know cone 4 refers to 1196C and cone 5 1222C because that tends to be around where I fire. Do note though that there are numbers with 0 before that refer to lower temperatures so don’t ignore a 0 to make sure you get the right cone. You can find more about pyrometric cones on Wikipedia.

Once you know your firing temperature other things to consider when choosing a glaze besides decorative choices like the colour, finish (gloss/satin/matte) and effect (smooth/speckle/opaque/transparent), are whether the piece is functional or not. If it’s functional, you need to think about things like whether your glaze needs to be food safe (most commercial glazes will say if it is on the pot, something like food safe or dinnerware safe); will it need to be cleaned so a smooth, easy to clean surface would be best; will it need to hold water/liquid so needs to be fully glazed inside, etc?

How a glaze is applied (method and thickness/number of layers) will affect the final outcome and most commercial glazes will offer suggestions as to the best application so I suggest reading the labels. The other thing about a lot of glazes, although not all, that is also worth bearing in mind is that not only the top temperature of the firing but how the kiln gets there will make a difference to the final outcome of a glaze. If you have access to a kiln so can control the firing program you can experiment with this as well as the thickness to see what differences it can make.

My tips for when starting with glazing would be:

  • Embrace the unpredictability!
  • Research firing temperature (and firing program if you can control that) to be sure you have the right glaze fit.
  • A good glaze notebook/app is worth its weight in gold.
  • Test first, either on tiles or test pieces.
  • Try to be consistent in how you apply and fire (but don’t get too hung up on this either).
  • But most importantly, have fun, play and experiment! You can get some truly amazing results.

To be comfortable and confident with glazing embrace the unpredictability of the glazing magic as well as the testing and note taking of the science. Enjoy and as always, if you have any questions feel free to get in touch!

If you’re interested in learning more about making in clay I have a range of video courses and workshops available in my shop.