A cram lesson in Crystalline Glazing

Crystalline Mug

 I had mentioned before that Crystalline firing is very involved…well, let me explain that a bit more.  It took me quite some time to figure out my own process and technique.  In my undergraduate studies I needed to explore my options in ceramics to see how I wanted to progress.  As I have said, I was really interested in crystalline glazing.  I had a bit of a block in my road when I asked about getting started with this particular glazing:  it had never been done at my school before and my professor had never tried it either.  At first I felt pretty discouraged, but then I thought I should just go for it!  I ended up buying a book on Crystalline Glazing to help me get started.  I read through as much literature as I could on the subject and looked at countless images of crystalline pottery.  I read all about formulating glazes, Ferro frits, oxides, color stains, the best way to sift a mixed glaze, and firing cycles.  I was truly overwhelmed with the information, but decided I just needed to pick a place to start and go from there.  So I began with the basic:  the form.  

Every piece that you throw actually needs three things – 1) the desired thrown vessel  2) a pedestal (short ceramic ring/cylinder) that is the same diameter as the base of your vessel  3) a collecting dish.  You can once-fire everything, but I have not had great success with that process, so I bisque everything.  Once all three pieces are completed and bisqued you have to stack them (for lack of a better word).  The pedestal gets glued to the base of the vessel – Elmer’s craft glue is actually perfect for this.  Then the vessel with the attached pedestal sits in the center of the collecting dish.  This is exactly how it will sit in the kiln during the firing process, and I will tell you why.  

Crystalline glazes are known for running off the pot.   There are some ingredients that will help it to run less, but it will almost always run off the pot.  That is why you need the pedestal (it protects the base of the vessle from getting coated in glaze).  The collecting dish does just what you would assume; collects the run-off glaze.  If you don’t use this dish, you are going to ruin your kiln shelves!  When your pieces come out of the kiln it is so exciting to see all the crystals, but then reality hits when you have to remove the vessle from its pedestal.  This is no easy task!  This is where you realize that crystalline glazing comes with a high mortality rate.  You can remove the pot by chiseling the pedestal clear off the base of the pot.  I do not recommend this way.  I lost the most pieces from chiseling.  If you have the resources, use a diamond-edged rotary blade power tool.  A hand-held one is fine.  This will actually give you a nice clean-cut and give you a better chance of saving your piece.  

Rutile Blossom

Ok, that is a lot to digest.  I’m sure you just want to know how do you get the crystals to grow?  Growing crystals is a matter of a couple of things:  your glaze and your firing cycle.  Crystalline glazes are made up of high percentages of calcined zinc oxide and a Ferro frit.  (Ferro frit 3110 is the best one in my opinion) These two ingredients is what really makes the crystals grow, but you also need the heat!  I have a pretty intricate firing process that takes between 19 and 22 hours to complete.  Once the kiln reaches its peak temperature (about 2310 degrees), it will hold for 4 hours.  It is in this holding period that the zinc is able to mature and form crystals – but it needs time to develop into large, noticable crystal formations.  

Crystalline Mugs

 After it holds for 4 hours, the temperature drops and there is another 1/2 hour to an hour hold.  Once this completes, the kiln will shut off and cool naturally.  Then I have to try to be patient while I wait with anticipation to open the kiln and see how the crystals did.  Sometimes I am stoked and other times I just want to cry when something didn’t  work.  It really is an amazing process, but you need a lot of patience if you want to get into this technique.  I think it is totally worth it!  If you really want to pursue crystalline glazing, I suggest you start by getting a book and start reading! 

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