Pugmills are often equipped with a vacuum pump and feature a chamber near the end of the barrel where the vacuum is applied to remove air (just before the auger that moves the material to the nozzle). The knives on the rotating shaft cut the clay within the chamber to expose as much surface as possible to the vacuum.
The de-airing process improves the smoothness of the fired surface
These two close-ups of a fired cone 6 porcelain showing a big difference in surface smoothness. The deaired material on the right has a much smoother fired surface even though the non-deaired material on the left has been wedged much more. The transparent glaze does not hide the roughness.
A low fire talc body lacks plasticity when slip-mixed, but not when pugged
This clay was slurried in a mixer and then poured onto a plaster table for dewatering. During throwing it is splitting when stretched and peeling when cutting the base. Yet when this same clay is water-mixed and pugged in a vacuum de-airing pugmill it performs well. One might think that the slurry mixer would wet all the particle surfaces better than a pugmill, but it appears the energy that the latter is putting into the mix is needed to develop the plasticity when there is a high talc percentage in the recipe.
Studio pugmills have come a long way
The same pugmill (back and front). One is stainless steel. Potters claim that they can dump almost anything into these machines (even dry scrap) and as long as they add the right amount of water the devices will mix and vacuum extrude a finished slug. Considering how portable these are they are an amazing device. However, these are no match for a large industrial pugmill. In the quantity of material they can produce, but also in the quality. They have few or even no blades on the main shaft, only augers. They contain no or only a rudimentary shredder feeding the vacuum chamber and little dwell time in the both chambers.
The large pugmill used at Plainsman Clays
The machine is being cleaned in preparation for a porcelain run. The machine has been stripped down completely and all the casings and augers and other parts have been washed and dried separately. These must be installed (in the main chamber, the vacuum chamber and in the nose). Clean-downs like this are an indicator of the quality delivered by the production crew.
Deairing pugmill at Plainsman Clays
The machine has been reassembled after cleaning and is ready for startup. This pugmill is powerful and capable of injecting alot of energy into the material. Premixed powder and water are fed into the main mixing chamber by a screw conveyor at the far end. Dozens of blades on the rotating shaft inside cut and mix the material so that by the time it has reached half way in the main chamber all traces of powder are gone. At the end of the main chamber an auger delivers the materials to a venturi terminated by a shredder. This slices the material with dozens of tiny blades as it enters the vacuum chamber (yellow cover). This exposes as much surface as possible to the vacuum. Additional blades on the main shaft further mix the material and finally an auger compresses it and delivers it to the nose where a column is extruded for cutting to length and packaging.