Arriving on foot to the Hanson Brick factory in Burlington, Ontario, Canada, the first thing I noticed was that the bricks were all the same colour on the inside, but they had an outside layer which varied from dark brown to white. My guide later explained that at this particular factory, they make 'coated' bricks, rather than 'through-the-body' bricks. The column of extruded mixture passes through a shower of sand, which has minerals in varying proportions to produce the final colour.
I'm not sure how much I like this idea, since I am trying to find out how a material is an expression of a place. In the case of Hanson Brick, the place, the quarry, is purposely suppressed for reasons of standardization and interchangeability. They are very particular about the colour of the brick, which has to exactly match sample panels which are kept at the very end of the production line- any brick that does not have the expected shade is tossed.
The quarry is definitely the biggest I've seen so far. In order to ensure a perfectly uniform mix, they excavate the full proposed depth and extract in vertical runs down the face, rather than risk running along the line of a sedimentary bed by going horizontally.
This raw material is shale, not clay. They have the same chemical composition, but shale is a sedimentary rock.
The shale is fed into a crusher which progressively screens and crushes it to a fine powder. There was a thick coat of light red dust on every visible surface. (See the footprints!)
The powder is emptied by conveyor belt into these large holding tanks. When the hoppers for the pugmill are mostly empty, a sensor automatically activates a second conveyor belt which feeds them.
The powder is mixed with more water, to about 12%, before being extruded. The extruded column passes through a series of dimpled rollers intended to add a bit of variation and relief in the surface.
The extruded column, or 'slug', is then doused with sand, cut, spaced, and loaded onto 'kiln cars' which pass first through a drying kiln, which uses the waste heat of the kiln to reduce the moisture content down to 0.5%, and then through the 180m-long tunnel kiln with a specific firing profile depending on the size of the brick. The maximum temperature is normally 1065C.
The bricks are then inspected and packed. The bricks produced at this factory are mostly for wholesale to builders, who might sign a contract for 4 million bricks per year.