Thank you Tom,   

I, personally,do not have pinholes when firing Stephen Hill's glazes on stoneware.   I use Cone 6 Pottery Supply House (in Canada) stoneware #519 and I have fired using both Stephen Hill and Ron Roy schedule.   However I have had this problem with other glazes and will now follow your instructions.

Denise:

I had the good fortune to spend three days with Ron Roy back in March, prior to the NCECA conference in Kansas City. Nothing like two clay nerds prating on for hours about formulation and firing. If I recall, Ron stated a peak of 2190? with a 30 minute hold in his book? We have since been emailing back and forth over PSD (particle size distribution) and flux levels in porcelain and stoneware bodies. Both bodies form a glassy matrix; but the mullite in stoneware requires a higher level of alumina to form properly.

I have been talking to the various clay mines in the States updating the data in my glaze calculator tables. There have been some changes in the last two years: both in iron levels and in naturally occurring potassium levels. Unlike kaolin in porcelain which have very little KnaO levels with the exception of one: most clays used in stoneware have a notable amount. These levels change as they dig through the various stratas of the mine. Although I cannot say with certainly, I am highly suspicious of these changes are playing a role in recent stoneware (pinhole) problems. Iron levels in two clays have increased by over 10%, so you will see that change also.

By stopping in the 2050F range, and slow ramping up to the 2180 +/_ range allows for more mullite formation, which thereby decreases absorption. This also allows for much more off gassing to occur before your glaze reaches its peak melt: thereby alleviating the problem of clay off passing being trapped in the molten glaze. However a word of caution: your hold time may need to be adjusted downward because the slow ramp will certainly increase heat work done.

The only time I look at glaze as being the problem child creating pinholes is when formula limits for KnaO are exceeded. The other is when extra thick applications of glaze are used to produce certain effects. Glaze recipes that have over 50% Nep Sy, Kona F4 (minspar) are automatically suspect for me. A 2 second glaze dip only amounts to 0.15-0.20 grams of glaze per square inch. In a typical application, off gassing of the glaze is rather minimal and short lived: always exceptions however.

Tom

Here is another example of pin holing:

Notice the discoloration around the pin hole?  The color is the same on the top half of this test cylinder.

The white glassy/glossy surface was in contact with the shelf. The upper part (left) did not fully mature. Notice the color is the same as the pin hole example above?  In porcelain, the fluidity of the melt is very similar to glaze because it relies on the same basic fluxes as glaze does. It takes more time to move through the clay body uniformly. Porcelain is KnaO/silica/metakaolin.  Stoneware is Al2O3/Silica- mullite. (spinel).

Tom