From: Gary Tefft / locustent@aol.com
Location: Menomonee Falls, WI
Date: 04 Jul 1999
Time: 01:34:30
The first thing I can tell you Nana is that your 20 Lb Butter Jar isn't salt-glazed. Salt-glazing is a method of creating a glassy coating on the surfacs of stoneware wy vaporizing salt in the kiln during firing of the ware. The salt reacts chemically with the clay to form sodium-silicate glass. The clorine from the salt escapes up the flue. Unless the clay contains more iron oxide than is normal for utilitarian stoneware, the glaze is essentially transparent and the color you see is the color of the fired clay. This might be nearly a battleship gray, if the kiln had a highly oxygen deficient atmosphere that reduced the iron oxides in the clay to metalic iron. Or, if the atmosphere was oxygen rich, or neutral, the color would more likely be tan. After the turn of the century stoneware manufacturers abandoned salt-glazing in favor of using blended ingredients including a fair amount of clay, flint, feldspar, zinc oxide and calcium carbonate. The flint, feldspar and clay melt to form a glassy coating to seal the surface of the ware. Both the zinc oxide and calcium carbonate act as fluxes to encourage melting. The calcium carbonate also provides opacity and the zinc a white coloration.
The 20 Lb Butter Jars, with their distinctive logo were introduced into the product line in the mid-teens, quite a while after the change-over from salt-glazing. They are found with small, as well as large wings, indicating that they were in the product line at least into the period in the mid-'20s when the wings on all ware got smaller. They are shown on pages 104, 144 & 152 of Red Wing Potters & Their Wares, which can be ordered from information on the referance page of this site.
-Gary-