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We now pass on to consider how it is that the vitrification of these forts has come about. Various opinions have been expressed:
(a) Some that it was done incidentally as the result of beacon-fires, or great fires for religious or other purposes.
(b) Other authorities see in these forts the intended result of structural operations, believing that the intention was to strengthen the parapet by fusing together the small stones of which it was composed.
The interdependence which can be observed in some of the groups of vitrified forts lends support to the view that they were used for signalling purposes, and I think that that may be assumed as certain, although there seems no reason to suppose that they only were used for that purpose any more than other forts in similar situations, which, being composed of different and more refractory materials, have not left the result of the fires so distinctly marked by the slag.
(a) I shall refer to the possibility of producing vitrification by beaconfires later on.
(b) Turning at present to the view that the vitrification was inten
tionally done to strengthen the parapet, we are met by some difficulties.
In the first place, it is almost certain that vitrification of the larger masses often met with, if intended as a structural method, must have been a troublesome business, and a process to which recourse would have been had only when ordinary building was impossible. But we find in various forts, notably at Tor Duin, near Fort Augustus, and at other places, that the loose stones below the vitrification are supported by ordinary masonry, which apparently might have been carried up the whole way had the builders so desired.
Another difficulty which presents itself is that the vitrification is not as a rule continuous all round the parapet, although for structural purposes, if that were the object, it would seem to be equally required at every point.
But not only is this the case, but we find that the greatest amount of the slaggy mass occurs often-I think I may say generally-where a strong parapet is least needed. At the top of an inaccessible cliff is often found the bulk of the vitrification. This is well seen at Shielfoot, Dunagoil and Ard Ghaunsgail (Arisaig). The last-named fort, which stands on a peninsula, has on the land side a defended entrance which would appear to be the weakest point; but the parapet is there devoid of vitrification, or nearly so.
It will also be admitted that, if the builders were determined to have solid walls, they were not very wise in setting them up on a foundation of loose stone, for they might have anticipated that the vitrified blocks would slip down the hill, as we find a great many of them have done, by the foundation sinking.
Lastly, I think it may be fairly assumed that, if the builders had designed a wall built with a mortar of semi-melted stone, they would have restricted its width to much less than a thickness of say 4 to 6 feet. In this connection, what are we to say to the mass of vitrified matter (described by Fraser-Tytler more than a hundred years ago)
extending along the east end of Craig Phadric, 40 feet wide and 70 feet long!
For these among other reasons, it seems unlikely that vitrification was undertaken as a structural method.
As the air of mystery still hangs over vitrified forts, it occurred to me that it could be to some extent dispelled if we could reconstruct a vitrified parapet; and my purpose to-night is to explain the experiments made with this object during the last five or six years, at long intervals and with insufficient leisure. The positive results have been poor, but a good many negative results have been obtained.
Beacon-fires seemed, at first sight, the most likely source of the necessary heat; so their results were first examined. Undoubtedly a large amount of slag can be obtained from burning grass or straw. This can be easily seen by inspecting the site of any large stack fire. I was fortunate enough (if I may use the expression) to see the results of a large stack-yard fire which occurred at Hay Mount Farm, near There fifty-seven stacks of grain and eight of hay were conIt was found, where the stacks had been recently erected and
the straw was strong, producing an open texture in the heap, so as readily to admit the air, as is the case with a rick of wheat, that there slag at the bottom. The silica had gone off in vapour, which partially condensed on the lee side of the rick in the form of pellets about the size of a pea. These could be found in considerable number on the ground. Where the material was closer in
as in the older oat ricks, some slag was seen at the bottom among the débris; but most of the slag was found at the bottom of the closely packed haystacks, where it lay in a nearly continuous crust, in places about 2 inches in depth.
I should have said that the wind was very high when the fire occurred, and that the combustion of the oldest stacks alone was at all slow.
The chemical analysis, by Professor Macadam, of the slag from this fire is given below, as it may be useful in further investigations :
was no had small
As the stack fires, when nearly burned out, had been extinguished with water and otherwise, and as the stone bottoming might have thus escaped fire action, the following experiments, among others, were carried out to ascertain the action of open fires having a base of selected stones. The first experiments were made on the high moorland near Riccarton. To begin with, on a base of stones constructed like a saucer, having a diameter of 5 feet, 100 stones of old moorland hay were burned. The process took about eight hours. The result on the stones was nil, but some very small streams of slag were found outside