Vitrified Forts – Lunar Society of Birmingham

Lunar Society of Birmingham

One of the great mysteries tucked away in a dusty corner of the academic archives [under layers of misdirection and credulous speculation] is the Vitrified Hill Fort.

The mystery dates back to the days of the Industrial Revolution, James Watt [of steam engine fame] and the Lunar Society of Birmingham.

James Watt FRS FRSE (1736-1819) was a Scottish inventor, mechanical engineer, and chemist whose Watt steam engine, an improvement of the Newcomen steam engine, was fundamental to the changes brought by the Industrial Revolution in both his native Great Britain and the rest of the world.


The Lunar Society of Birmingham was a dinner club and informal learned society of prominent figures in the Midlands Enlightenment, including industrialists, natural philosophers and intellectuals, who met regularly between 1765 and 1813 in Birmingham, England.

At first called the Lunar Circle, “Lunar Society” became the formal name by 1775.

The name arose because the society would meet during the full moon, as the extra light made the journey home easier and safer in the absence of street lighting.

The Lunar Society evolved through various degrees of organisation over a period of up to fifty years, but was only ever an informal group.

While the society’s meetings provided its name and social focus, however, they were relatively unimportant in its activities, and far more activity and communication took place outside the meetings themselves – members local to Birmingham were in almost daily contact, more distant ones in correspondence at least weekly.

In 1777 the Scottish mineral surveyor John Williams exchanged letters with James Watt regarding the “curious” vitrified granite walls of Craig Phadrig hill fort overlooking Inverness.

And Watt’s own geological skills had grown since his early surveying days.

In 1777 he supplied notes and sketches to his fellow countryman John Williams, and engineer, antiquary and geologist, who had asked about the curious construction of the fort of Craig-patrick near Inverness, which Watt had studied during his canal surveys.

The local rock, Watt told Williams, was a granite mixed with quartz and the fort’s walls had been made by burning this, melting the granite into glassy cement around the the hard quartz.

The Lunar Men: The Inventors of the Modern World 1730-1810
Jenny Uglow – 2011

Craig Phadrig is a forested hill on the western edge of Inverness, Scotland.

The summit of the hill is occupied by a vitrified fort; a stone structure affected by fire to produce a glass-like material.

Radiocarbon dates obtained in the 1970s suggest that the inner wall was constructed in the 4th century.

Excavations at this time revealed evidence of Pictish occupation, including metal-working implements and French pottery, dating from the 7th century.

Craig Phadrig

And for the last 239 years the mainstream has been sporadically busy discussing vitrification temperatures whilst trying to figure out a viable technique for melting granite hill forts.

The melting temperature of dry granite at ambient pressure is 1215–1260 °C;

Pour fondre de tels murs de granite, une température supérieure à 1100 et 1300 °C est nécessaire et doit être combinée à un procédé de combustion lente.

Granite, basalt, gneiss or other silicate rocks begin to crystallize at temperatures about 650°C, and melt and vitrify when exposed to temperatures between 1050 and 1235°C.

Biotite micas melt at 850°C.
Carbonate rocks such as limestone and dolomite become calcined when exposed to temperatures of 800°C. – Vitrified Forts

Temperatures in excess of 1000 degrees Centigrade would need to be applied consistently over large areas of the wall, in close proximity for a significant period of time.

Brigantes Nation – Vitrified Forts

So whilst the modern mainstream are busy going nowhere the inquisitive reader is advised to read an account of a Vitrified Hill Fort exploration written by John Williams in 1789.

The Natural History Of The Mineral Kingdom -  Volume 2 - 1789 - John Williams
I saw the ruins of several ancient vitrified forts in the Highlands of Scotland, situate upon the level summits of small hills of the breccia.

There was a section made, a few years ago, quite through one of those singular forts, beginning without all the ruins, and digging to the rock, and keeping down to the solid rock, until they came to the vitrified wall that surrounds the level area upon the top of the hill, which was the area of the fort.

They continued the cut to the rock through that wall, through the middle of the area of the fort, and through the opposite wall.

There was also a section made half way thro’ an out-work, which defended the principal access to the fort.

The person who attended the cutting of those sections, was as inquisitive as the Sicilian gentlemen possibly could be ; and he noted down every discovery and observation he made upon upon the spot.

I need not observe that the ruins of those forts have a near resemblance to lava, and in fact are lava ; the whole wall being vitrified and run together by the force of fire into coarse glass or lava.

Now, it is a fact worthy remarking in this place, that in making the sections above-mentioned, the parts of the lava or vitrification which were most exposed to the external air, were by far the hardest, in so much, that in going through the ruins of the south wall, where large fragments were exposed to the air without any manner of cover, the workmen could not break some of them ; but were obliged to roll them out along the trench with levers, until they tumbled over the brow of the hill ; and they were obliged to dig a passage under one fragment, which was too large to be removed, and leave it like a bridge over the cut ; on the contrary, the north wall, though much deeper and broader than the south, was nevertheless much easier dug through, it being covered with rubbish, which had fallen upon it from a row of houses built within the fort against this wall.

The north wall had been as well vitrified as the south to the full ; yet was it easier cut through, being covered from the external air.

The vitrified ruins of the houses which had been reared against the north wall facing the south, were still more decayed than the wall, so as to be easily broken to pieces, and they were buried much deeper under rubbish and earth.

The ruin of the north wall was about fourteen feet high or deep, of clean vitrified matter, and very broad, the top of the wall being fallen down ; but the ruin of the out work was about twenty-four feet high, though all was fallen in and crushed together.

This great heap of ruins produced some vitrified fragments on the out-sides ; but when they entered a little way in with their section, they soon found more, which increased as they advanced, and before they were one third of the way through, there was nothing found but vitrified ruins from top to bottom, except about a foot of soil and rubbish which lay above the whole.

As they advanced in with the cut, they met with many large fragments, which had been fully as well vitrified as any part of the walls ; they were, nevertheless, all very easily broken, and those which were high up fell to pieces with their own weight, when they tumbled down.

Now, in all these experiments, it was found, that the more the lava of those extraordinary ruins was covered from the external air, the more it was found in decay.

The one end of the hill, which was the site of this fort, ended in a narrow ridge, upon which there was the ruin of a long narrow out-work, stretching in a right line from the fort.

This never had been above eight or ten feet broad.

There is the clearest vestigia of their having a draw-bridge about the middle of it.

There is a gap in the ruin, and a ditch in the ground where the bridge has been, to prevent access past the bridge to the surrounding wall of the fort.

It is natural to reflect, that as this wall or out work was so narrow, much of it would be exposed to the air, and all the changes of the weather, which is pretty severe in that country ; it being situate about two miles west of Dingwall in Rossshire, and about nine hundred feet of altitude above the valley immediately at the foot of the hill.

Now, it is remarkable, that this narrow out work, though most of all exposed to the air, is still more entire than any other part of the ruins, which corroborates the foregoing observations, and increases the doubt of the lava of volcanoes decaying first at the surface.

And moreover, I saw one of these curious ruins within two miles of Inverness, where there had been an outer and an inner fort all vitrified.

The inner surrounding wall, which has been by much the highest, is now nothing but a heap of rubbish mixed with vitrid fragments.

The outer wall, on the contrary, never was high, and of consequence, not so apt to tumble down by its own weight in length of time ; of course, it would continue to be exposed to the air and weather, and there is now some part of this wall standing entire upon the bare rock where it was first founded.

I look upon this entire piece of the remains of these extraordinary ruins as one of the greatest curiosities in Europe.

There was a pamphlet wrote some time ago, containing sundry observations about the discovery of those ancient remains ; and from traditions picked up, it is supposed in the pamphlet, that the one near Dingwall was the ancient Selma so often celebrated in Ossian’s Poems.

Be that as it will, these ruins are certainly very ancient.

It is near two thousand years since the Romans landed first in Britain, after which period, there was no occasion for such extraordinary shifts in cementing strong walls, as the use of lime in building was about that time introduced into the island.

The Natural History Of The Mineral Kingdom – Vol 2 – 1789 – John Williams

And in the last 239 years a lot more Vitrified Hill Forts have been discovered…

Tap o' Noth

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