Construction methods in the nineteenth and early twentieth centuries
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Figure 3.21 Damage caused to tiles under tension by floodwater, the Hallway, Osgoode Hall, Toronto.
use of Portland cement as an adhesive. Moisture is absorbed into the body of the tile and trapped by the Portland cement adhesive layer; the ‘freeze thaw cycle’ and the different thermal expansion rates of the glaze, clay body and cement cause a separation between two or all three of those elements resulting in delamination of the glaze surface, and fracturing and falling away of whole or part tiles from the cement layer (Figure 3.22). External tiles which have survived unscathed have done so only because their location is frost protected in some way. Mortars and adhesives for resetting nineteenth and early twentieth century tiles As we see nineteenth century sources recommended mixes for setting tiles with ratios of sand to cement which gave a particularly dense, non-porous, and hard final set. Nevertheless during the same period many technicians used an all-purpose building mortar mix, one part Portland cement, one part hydrated lime to six parts well-graded sharp sand, for general construction purposes other than tile setting; the cement to sand ratio recommended by the tile manufacturers was exceedingly hard by comparison. Hydrated lime is calcium hydroxide in powdered form and is produced by the controlled slaking of quick lime with steam or minimum amounts of water. It is generally much less reactive than lime putty and it is non-hydraulic. The hardness, non-reversibility and sometimes damaging effects of historically correct cement and sand mixes as recommended by the nineteenth century tile manufacturers, and which found widespread use, present the conservator of Victorian floor and wall tiles with the dilemma of using the historically correct mix which will replace like with like, or changing the adhesive mix to ratios closer to those acknowledged in