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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.
Prevailing knowledge and opinion is that deaired clay normally has better forming properties and produces a smoother fired surface than that prepared by other methods. This is especially true for bodies of lower plasticity or of certain formulations (e.g. high silt, high talc). However for most plastic terra cottas, stonewares and porcelains; slurrying, dewatering and wedging produce similar workability.
Traditions in many places are to age clay after pugging to improve plasticity. However on closer examination it becomes evident that the body has low plasticity, any increase is considered worth the effort. However for plastic bodies (e.g. those used on a potters wheel to make large ware), the clay is fine right out of the nozzle of the machine. Today, low plasticity is normally managed by a simple bentonite addition or substitution of kaolin for ball clay.
Pugmills can be a part of a larger body-making process or it can be the only one. Theoretically, the ideal is a slurry mixing process to thorough blend the materials and wet all particle surfaces (called blunging). The blunger then feeds a screening device that removes coarser particles. That in turn feeds a filter press that dewaters the slip. Flat filter cakes produced by the press are then fed into a premixer that re-blends the separated layers in the cakes. The premixer then feeds the pugmill and it finalizes mixing and de-airs and extrudes the material. However the more plastic the clay the less less practical are some of the steps in the above process. This is because highly plastic bodies do not screen well, they do not filter press well and they stick inside the premixer. If a de-airing pugmill has enough blades and they are angled for the best mixing possible (rather than speed), the dry material and water can be fed directly into the head of the pugmill chamber and it can do all of the steps by itself. For super plastic bodies there is not really another practical method.
Studio potters often use de-airing pugmills to re-pug incoming material and to reprocess production scrap. Very capable small pugmills are commonly available and potters highly value them.
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 can dump almost anything into these machines (even dry scrap) and as long as they add the right amount of water these devices will mix and vacuum extrude a quality finished slug. Considering how portable these are they are an amazing device.
The large pugmill used at Plainsman Clays
All equipment has been cleaned in preparation for a porcelain run.
Deairing pugmill at Plainsman Clays
By Tony Hansen