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Secondary Clay


Clays that have been transported by water from the site of their primary alteration and settled into layers are called "sedimentary" (as opposed to "primary" clays which are found near the site of alteration). Sedimentary clays are classified according to the geological period in which they were formed. Nature purifies these clays mechanically, removing gravel and sand. But it also contaminates them chemically or at the ultimate particle level, adding iron and fluxes. Vast deposits of secondary clays can be found in many places and materials having a wide range of properties can layer right next to each other. Ball clays, earthenwares and stonewares are secondary clays.

Mel Noble at Plainsman Clay's Ravenscrag, Saskatchewan quarry

Mel Noble at Plainsman Clay's Ravenscrag, Saskatchewan quarry

Plainsman extracts 6 different sedimentary clays from this quarry (Mel knows where the layers separate). The dried test bars on the right show them (top to bottom). The range of properties exhibited is astounding. The top-most layer is the most plastic and has the most iron concretion particles (used in our most speckled reduction bodies). The bottom one is the least plastic and most silty (the base for Ravenscrag Slip). The middle two are complete buff stonewares made by mother nature (e.g. M340 and H550). A2, the second one down, is a ball clay (similar to commercial products like OM#4, Bell). A2 is refractory and the base for Plainsman Fireclay. The second from the bottom fires the whitest and is the most refractory (it is the base for H441G).

IXL Industries clay quarry near Ravenscrag, Saskatchewan in 1984.

IXL Industries clay quarry near Ravenscrag, Saskatchewan in 1984.

Layers of the Whitemud Formation are being mined. The layer being extracted is a silty stoneware they referred to as the "D member" (equivalent to Plainsman 3D which is mined several miles to the east). Below the D they continued to mine a much whiter kaolinized sand of equal or more thickness. Above the D is a ball clay (equivalent to Plainsman A2). Above that is a light burning stoneware (the combined layers that Plainsman extracts separately as A3 and 3B). A foot-thick layer of much harder volcanic ash is visible in the green over burden at the top. From these stoneware clays they made brick of exceptional quality, firing it as high as cone 10. Twenty years later the company reclaimed this land and today you would be unable to find where the quarry was located.

Out Bound Links

  • (Glossary) Stoneware

    Most often the term stoneware refers to a high fired (about 1200C+) ceramic clay:feldspar:quartz blend that is semi-vitreous (not translucent and not zero porosity). To appreciate the scope that stoneware can encompass it is helpful to contrast it with porcelains (this description is for people who ...

  • (Glossary) Earthenware

    A clay fired at low temperatures (cone 010-02) where it does not develop maturity (vitrify). The term earthenware almost alway refers to red burning terra cotta ware (although it is a somewhat more general term referring to a wider range of colors and more primitive forming and firing techniques). E...

  • (Glossary) Terra cotta

    'Terra Cotta' (Italian for 'cooked earth') is red burning earthenware. It has been made for thousands of years by indigenous cultures, most often unglazed. If glazed, high lead content mixtures have been traditional. It is fired at much lower temperatures than stoneware so, not surprisingly, it is n...

  • (Minerals) Ball Clay

    Ball clay is the most common type of secondary clay. They are much more plastic than kaolin because the particle size is much smaller. Ball clays are available with a wide range of plasticities (a pro...

  • (Glossary) Primary Clay

    Kaolinite is the closest thing we have to pure clay mineral. Clays that have been deposited at or near their site of alteration. They typically are contaminated by rocks and mineral particles that must be removed by processing. The cleanest and lowest iron clays are from this type of deposit. Kaolin...

In Bound Links

  • (Glossary) Fireclay

    A refractory naturally occurring secondary clay. Fireclays are refractory because they contain high concentrations of Al2O3 and low concentrations of fluxes (like Na2O, K2O, CaO, MgO). Kaolins actually qualify as a super-duty fireclay because they contain almost no fluxing oxides (and are thus very ...


By Tony Hansen




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