Dave Finkelnburg on sat 17 feb 01
You said your oil lamps weep a bit of oil. I am sorry to hear that.
Basically, what's in the archives is oil can get through much smaller
passages than water. A water-tight container may not hold lamp oil.
As you recognize, the solution is to get the clay sufficiently vitrified
to hold the oil in. That means fusing the clay better than you have.
"Better" is totally subjective. Only you can measure how much your clay is
vitrified. You have evidence that so far it's not vitrified enough.
Firing higher is clearly in order. How much higher could best be
determined by testing. You need to test for absorbtion of water. You need
to fire unglazed test tiles in your glaze firing to make the absorbtion
tests. It would be best to get some measure of the absorbtion of your clay
at the cone you are currently firing to, so you know how high the absorption
is in your current lamps.
Also, remember that one test tile will not represent your whole kiln
unless your kiln fires very evenly.
Keep in mind that the glaze alone will probably not do the job for you.
If the clay is porous, the glaze probably won't make up for it. Get the
clay vitrified enough, then fit the glaze to it, then you'll have lamps that
make you happy.
Dave Finkelnburg in Idaho