London Stock bricks

Alan Cox 1997.  A vital component: Stock bricks in Georgian London.  Construction History 13, 57-66.

The London Stock is a type of brick the manufacture of which is confined to London and south-eastern England (particularly Kent and Essex). It is made from superficial deposits of brickearth (Loess) overlying the London Clay, which are easily worked and produce a durable, generally well-burnt brick. This durability actually increases, since the London Stock brick has the fortuitous advantage of hardening with age and in reaction to the polluted London atmosphere..

Other characteristics of the London Stock result from its method of manufacture, two stages being especially important. The first of these is the practice of mixing the clay with what has been variously known as Spanish, soil, town ash, or rough stuff- that is London's domestic rubbish, which contained a large amount of ash and cinders. The addition of this sifted ash provided a built-in fuel when the bricks were fired..

I.L.Freeman 1964.  Mineralogy of ten British brick clays.  Clay Minerals Bulletin 5, 474-486. 

Sample 63AH; London Stock brick mixture; mainly Pleistocene from Kent.  75% brickearth, 10% estuarine mud, 10% washed chalk, 5% sifted town refuse. The chalk is added to produce the desired yellow colour. Freeman carried out thermogravimetric analysis on his brick clay samples using a Stanton Redcroft TR01 thermobalance.

The TG curve for 63AH suggests not much moisture loess at relatively low temperatures, this would be expected for a material with a relatively low clay mineral content. This is also reflected in the low weight loss at dehydroxylation temperatures. Greater weight loess occurs at higher temperatures where there may be decarbonation reactions from the chalk in the system. 62AK has larger weight losses at moisture loss temperatures and dehydroxylation temperatures indicating a larger clay mineral presence.

This is a very rough first-plotting of the DTG curve for the Freeman sample 63AH- the brickearth mixture. It shows two main thermal events (c.f. the DTA results above); four events can be tentatively identified: A the loss of adsorbed water- not very much water, not strongly held; B combustion of organic material, again not a great reaction, not a lot of organic material present; C clay mineral dehydroxylation, the classic clay mineral reaction, showing a modest amount of clay mineral material in the sample; D carbonate loss CaCO3 >  CaO +  CO2, the added chalk has an effect here, not a huge reaction, only a small % of chalk added. The whole 63AH DTG curve looks quite like the DTG curve for a Canadian quickclay; the dominant quartz of course offers no analytical signal.

For more on DTG applications see: Fordham, C.J., Smalley, I.J.  1983/4 High resolution derivative thermogravimetry of sensitive clays.  Clay Science 6, 73-79.