Your guess about the silica is correct. Silica is the ingredient that
makes the glaze glossy. The more silica, the more glossy the glaze.
However, too much silica and your glaze will run right off of the pot
and onto your shelves. Kaolin, ball clay, and Alumina are
ingredients that are frequently used in glaze to matt the glaze and
raise the flux temperature, An underglaze is basically stains or oxides
for color, some clay, and just enough silica to hold the mix together
and make it stick your clay in the firing process. If you overfire
underglazes, they begin to get shinier. Sometimes I have purposely
overfired underglazes to get a soft sheen to the color without using
glaze. If a glaze mixture that you concoct or buy is too runny at the
temperature you want to fire to, you can add clay or alumina to raise
the flux temperature. The one thing I can't remember much about is
feldspars in glazes. Potash, etc. Maybe someone else can jump in here
to help more. I had a HORRIBLE glaze chemistry instructor. My worst
memory in college. He gave us ONE lecture. We read a thick and
dificult book on glaze chemistry and we had an open book test at the
end, based strictly on the book. We did experiments on our own in
between. PATHETIC. I remember that though we were given instructions
not to collaborate on the exam, several people did. One gave the exam
to her chemist husband to look over. He said some of the questions were
impossible to answer. I squeaked by with barely a C. He gave us
equations and formula related questions that needed to be worked
backwards first so that you could get your answer, find missing links,
etc. We were all so angry. He was head of the ceramics department,
too. Cruel is the word I would use to describe him regarding this
class. NOT teacher. However, he certainly could teach you everything
else about clay! We built kilns, fired like pros, and I learned to
throw and handbuild large pieces under his instruction. High fire
reduction firirin, salt firing, ash kilns and glazes, and raku firing,
and pit firing were a blast. Funny, we learned NOTHING about low fire
glazes, but in an electric kiln it's so predictable there isn't too much
to stress over. I love low fire glazing and underglazing now...love
color and pattern in my current work. Thank goodness there were some
fabulous grad students in our ceramics department. I learned as much
from them as anyone.
Linda Woods,
Visit our Students' Art Gallery at St. John's School: www.sjs.org,
click on Fine Arts, Click on Lower and 6th Grade Art Gallery.
