The new building at 22 Gordon Street houses The Bartlett School of Architecture. Completed in 2017 and designed by Hawkins\Brown Architects, the building has recently been named on the shortlist for The Architect’s Journal AJ100 Building of the Year prize and the RIBA London Awards. The structure is a retrofit of the former Wates House, the ghost of which still exists in the core of the building. What replaces it is a spacious, fit-for-purpose building for the UCL School of Architecture.
Now those of you that know me and know that my interests in the fabric of UCL are mainly biased towards the use of stone, may have assumed that I was disappointed to see a brick and concrete structure appear on the corner of Gordon Street and Endsleigh Gardens. Not so. I declare a fondness for both materials, though I confess to prefer my concrete to be around 2000 years old, the bricks used to clad 22 Gordon Street are rather special.
I am very grateful to the Bartlett’s Kevin Jones for supplying me with a specimen of ‘The Gordon Street Klinker’ a brick made specially for the construction of the building by the German brick makers Janinhoff.
Janinhoff make a great deal of bricks of different compositions, but mainly to a standard size. The Gordon Street Klinker is slimmer and longer than the standard bricks in their repertoire. 140,000 of these bricks, measuring 290 x 52 x 70 mm were used on the façade.
The brick is water-struck and twice fired. The water-struck brick making process works well for high moisture content, high plasticity clays. The clay is ‘struck’ in a wooden mould, and the large amount of water present allows for the mould to be easily removed, without the clay sticking to it. Evidence of this process can be seen in the puckered, ‘troweled’ surface of these bricks and the lip from clay overhanging the top of the mould. Lower water content clays need to be ‘sand-struck’; i.e. the mould is coated with sand which stops the brick from sticking to it.
Evidence that these bricks are hand made can be seen in this example from the Endsleigh Gardens façade of the Bartlett’s 22 Gordon Street Building …
Look closely and you can see the brick maker’s fingerprints …
Janinhoff fire their bricks in a circular, so-called Hoffman Kiln which can operate continuously. Patented by Friedrich Hoffmann in 1858, these are the standard brick kilns used worldwide today. They are large circular or oval structures often with a central chimney. The interior has firing chambers radiating out from the central space (below the chimney). The kilns are fired by a movable ‘fire wagon’ which can travel on rails around the interior, firing each chamber consecutively.
The Gordon Street Klinker is coal fired at maximum temperatures of 1200°C. Looking at the surfaces of the bricks, ‘firing ghosts’ are seen patterning the surface of the bricks. This tells us how they were stacked in the kiln, subtly changing the oxidation of the brick surfaces where they touch each other.
Above: Janinhoff’s Hoffmann kiln, showing bricks being stacked ready for firing and also note the use of spacers and other kiln furniture.
The grey colour of the bricks indicates a low-iron content clay. Such materials are available in the northern Germany and Denmark deposited as Quaternary glacial clays. The Jutland Peninsula lay at the southern edge of the ice sheet during the last glaciation and the clays were deposited as the ice sheet retreated between 20 – 10 thousand years ago.
To be workable, reduce shrinkage and cracking on drying and firing and be strong, brick clays need to contain inclusions or temper. Inclusions are natural mineral grains that occur naturally in the clay whereas a temper is added by the brick makers and can include both mineral and organic particles.
To look inside the Bartlett bricks it is necessary to produce a thin section, a slice 30 µm thick for observation using polarising light microscopy. This technique is routine in geology for examining rocks and identify their component minerals. As ‘synthetic stones’, ceramics may also be analysed in this way under the discipline of ceramic petrology.
Under the microscope, the Bartlett bricks have an almost isotropic (opaque) clay matrix indicating that the clay minerals have broken down and have begun to melt. This indicates that the brick has been fire in excess of 1000°C (and indeed we are aware from the manufacturers that temperatures of 1200°C were attained). It is not possible to distinguish whether or not the brick clay contained inclusions or it has been tempered, or perhaps both! Mineral particles present are dominantly quartz, but chert, feldspar and zircon are also present, indicating granitic rocks in their source. Grains are poorly sorted ranging from very fine (a few 10s of microns diameter) to particles of around 0.5 mm, the latter just visible to the naked eye. It is reasonable to expect that the finest portion are naturally occurring inclusions in these glacial clays.
The photomicrographs below show the brick photographed in plane polarised light and under crossed polars, x 40 magnification.
The mineral grains are sub-rounded to sub-angular, indicating a natural sand source (i.e. not mechanically crushed). The composition is clearly granitic as indicated by the feldspar, zircon (this in typically tiny grains) and the large quartz grains with fluid inclusion trails. Some of these features are seen in the photomicrographs below. Left, the large grain in the centre is a feldspar; x 40 magnification, the field of view is ~ 3.5 mm. Middle and Right, quartz grains with inclusion trails, x 100, the field of view is ~ 1.5 mm.
Siddall, R., 2017, Inside the Bartlett Brick, Blog: Orpiment https://orpiment.wordpress.com/2017/06/02/inside-the-bartlett-brick/