The hydraulic properties of the concrete used in the floors of Roman hypocausts, has, I believe, escaped the notice of previous writers, and is the only other point which need longer detain us in this interesting building. My attention was drawn to this subject by my brother, Mr. George Barclay Bruce, Member of the Institution of Civil Engineers, in the following communication:—
In many places on the line of the Wall, the mortar has had mixed with it broken tiles or burned clay, to assist it in resisting the moisture of the atmosphere.
The concrete at Chesters placed between the slabs of the bath-room, has a very large proportion of this burned clay, and would thus be better suited to resist the action of heat below and water above than purer lime.
A portion of this concrete was taken, by way of experiment, and burned in a crucible, as though it had been a piece of limestone; it was then ground fine, and mixed with a proper quantity of water; after being allowed to dry for three or four hours, it was immersed in water, where it set in the same manner as common mortar does in the open air, clearly proving its hydraulic properties. The same experiment was tried with the ordinary mortar of the Wall, but without the same result, there not being a sufficiently large quantity of burned clay to enable it to stand so severe a test. In the case of the concrete, it did not set so readily as what is called Roman cement, but sufficiently so to prove that it is a strong hydraulic mortar, made by the mixture of burned clay with common lime.
Bidding farewell to these interesting structures, we may now bend our steps a short way down the river, on a visit to the cypress-grove—the burial ground of the station. This, which in Horsley’s days formed a separate field called the Ox-close, is now included in the park of the domain. Never was spot more appropriately chosen. The river here descends with more than usual rapidity over its stony bed, and bending at the same time to the left, exhibits to the eye the lengthened vista of its well-wooded banks. No earthly music could better soothe the chafed affections of the hopeless heathen mourner than the murmur of the stream which is ceaselessly heard in this secluded nook. From this spot have been procured several sepulchral slabs which will presently afford us instruction; meantime, one is given on the next page whose lesson is of a negative character. The blank memorial shews how vain are the efforts which even affection makes to render buoyant on the wave of time the memory of those departed. Our very monuments need memorials. But, passing this, the character of the carving betokens a poor state of the arts, and fixes its date in the lowest times of the empire: in this we have a proof of the long-continued occupation of the station. The fate of the stone has been singular. When Horsley saw it, the inscription was legible; but having since been used as the door-stone of the cow-house at Walwick Grange, the letters had, previous to its removal to Alnwick-castle (its present resting place), been entirely obliterated.