Clays and bricks of the penal settlements at Port Arthur and Maria Island, Tasmania

The making of clay bricks was a major industrial activity at the penal settlements of Port Arthur and Maria Island. The silicate minerals in the clays used were mainly illite and kaolin as is usual in brickmaking. All bricks examined had more than 1.2% of Potassium Oxide and so the surface clay at the Port Arthur settlement, which is montmorillonitic and low in potassium, was not used. Many of the bricks made at Port Arthur are soft due to underfiring and unaltered clay minerals have been detected in some.
Much of the brickwork of the ruins at Port Arthur has disintegrated due to the lack of strength in the bricks and due to the presence of more than 1% salt, mainly NaCl, which, as the result of exposure to rain wind and sun, has crystallized near exposed surfaces. Bricks, collected from inside a continuously occupied building and thus not exposed to any source of salt or water, were sound but contained 1 2% salt. As the salt is similar in composition to that of seawater, it is concluded that at least some of the bricks were made from clay that had been puddled with seawater.
An old photograph of the Penitentiary with its roof still intact and hence taken before the 1898 bushfires shows deterioration of the exposed bricks at that early date. It would appear that a large number of the bricks manufactured at Port Arthur during occupation of the settlement were of such inferior quality that they could not be expected to last for centuries.