Workers thus had to find a way to remove impurities in order to achieve a good fired surface. Highly liquid clay slurries were the answer since they could easily be screened to remove undesirable particulate material and run through magnetic separators to remove metal particles. However large quantities of water then had to be removed to return the clay to a pliable plastic form.
These machines had precision flat cast iron plates that fit precisely against each other on their outer perimeter but formed a hollow interior. These were placed against each other on a long rack with a thick canvas layer between neighboring plates. The rack was then compressed together and held securely by tightening a large center bolt. The slip was then pumped into the inside chamber under great pressure. The canvas-filled gaps between the plates provided a channel for the water to escape. Finally the whole press was disassembled and 'cakes' of water-reduced maleable clay were extracted.
The filter pressing process produced some material segregation and stiffness variation so remixing was required. You saw a green pugmill outside the kiln at the beginning of the tour. Pugmills mix and extrude a clay column of the desired size. We have a horizontal and vertical pugmill on display in this room. The horizontal pugmill was built around 1910 and its sister machine is still in use at Plainsman Clays across the street from this plant. Note also the stack of filter cakes that remain since the plant closed.
The most skilled and creative workers in the company worked in this department. The mould storage room contains master moulds going all the way back to 1938, many of these could never be replaced if lost or damaged. Mould makers used turning and lathing equipment and carved free hand to make solid plaster models and then master moulds of each new product. All of this was done without the rubber and plastic materials used commonly today.
The master moulds were employed to make the hundreds and thousands of working moulds that were used in the plant. Each would typically last for 200 castings before ware quality suffered to the point that it had to be replaced. The mould making department was thus kept very busy and played a big role in overall product quality.
Refractory ceramic saggars were commonly employed in the potteries in Medicine Hat. However in industry today they are seldom used because of the tremendous energy required to heat the extra bulk in the kiln. Please proceed to the kiosk 7 and you will learn more about what saggars are and why they were used in this plant and others.
(master and working moulds on display at the kiosk)
