MANUFACTURING MATERIALS & PROCESSES
The manufacture of saltglaze stoneware was long established in the European lowlands, but the process was only developed in England during the mid/late 17th Century, primarily by John Dwight at the Fulham Pottery, whose work is well documented in other publications.
The body of the wares was formed from a clay mixed with ground flint, which was either turned on a wheel, or formed in a mold.
After the form was created, the desired details were impressed into the clay when it was in a plastic state, sufficiently firm not to collapse under the pressure of the lettering being impressed, yet still soft enough to receive the impression of the lettering.
Once air drying had been completed, the wares were then placed in a downdraft kiln, where the temperature was brought up to a maximum of 1300 deg.C, which would cause the wares to vitrify. At this point ports around the kiln were opened, and a quantity of salt was poured in, which would vapourise, reacting with the surface of the wares to form a thin hard coating. In the absence of temperature monitoring equipment, estimation of kiln temperature would be largely down to observation of heat colours within the kiln, viewed through the ports.
Firing of salt glaze kilns was a pretty nasty business, and apart from the quantity of smoke produced from the burning coal, the introduction of salt into the kiln led to the discharge of hydrogen chloride gas, which when mixed with water would produce hydrochloric acid. This would have made the surrounding areas very unpleasant to live in!
In his book, J F Blacker gives a very graphic account of witnessing the firing of a saltglaze kiln, which is well worth reading.
The body of the wares was formed from a clay mixed with ground flint, which was either turned on a wheel, or formed in a mold.
After the form was created, the desired details were impressed into the clay when it was in a plastic state, sufficiently firm not to collapse under the pressure of the lettering being impressed, yet still soft enough to receive the impression of the lettering.
Once air drying had been completed, the wares were then placed in a downdraft kiln, where the temperature was brought up to a maximum of 1300 deg.C, which would cause the wares to vitrify. At this point ports around the kiln were opened, and a quantity of salt was poured in, which would vapourise, reacting with the surface of the wares to form a thin hard coating. In the absence of temperature monitoring equipment, estimation of kiln temperature would be largely down to observation of heat colours within the kiln, viewed through the ports.
Firing of salt glaze kilns was a pretty nasty business, and apart from the quantity of smoke produced from the burning coal, the introduction of salt into the kiln led to the discharge of hydrogen chloride gas, which when mixed with water would produce hydrochloric acid. This would have made the surrounding areas very unpleasant to live in!
In his book, J F Blacker gives a very graphic account of witnessing the firing of a saltglaze kiln, which is well worth reading.