Growing pains – Fireproofing buildings using ‘Flower pots’

Belper North Mill is a structure of world importance as the second oldest surviving iron framed building in the world and the oldest with an iron framed roof and also having the oldest surviving lift shaft. This revolutionary design was part of a long process of developments to produce a building that would be resistant to fire, a fate that befell its predecessor. This month ‘Our Favourite Things’, an exploration of some of the more unusual objects in the museum collection of Strutt’s North Mill, continues with the origins of the clay ‘flower pots’ that were a key part of the North Mill’s structure and of early attempts to construct fire proof buildings. These pots were nominated by our previous museum Manager Mark Higginson, who explored their history as part of his extensive work to develop new interpretation for the ‘Cave’ of the North Mill.

The flower pots owe their origins to a problem faced by early industrialists when constructing and operating mills in the late 1700s. These mills had an alarming propensity to burn down as they were often packed with flammable materials. The traditional construction methods of many mills, with stone or brick walls supporting a wooden internal structure of beams, joists and floors, made them extremely vulnerable to devastating fires.

These would leave little more than a burnt-out shell of the building often putting the mill owners out of business, as most mills were not insured against fire, and leaving the workers without jobs and destitute in a time before any kind of state welfare was available for the unemployed.

Fire at the Albion Mill by Thomas Rolandson

The Albion Flour Mill, one of the most technologically advanced mills in the world was gutted by fire March 1791. Disasters like this set the mind of enterprising industrialists working on finding new ways of reducing the risk of fire in their mills.

In order to try and prevent this, mill owners, including the Strutt Family, began exploring ways to make their mills fireproof by removing or protecting as much of the combustible material of the buildings structure as possible. William Strutt (1756-1830), son of Jedediah Strutt founder of the Belper mills, was instrumental in finding ways of fireproofing buildings. He was involved in the construction of a series of mill buildings which included features to make them resistant to fire. His earliest attempts involved cladding the structural wooden timbers first in plaster and then metal sheeting, as used in the former Belper West Mill (1795), gradually evolving to the use of metal for the internal structure of columns and girders as used in Belper’s rebuilt 1804 North Mill.

To replace the exposed – and flammable – wooden floors that were supported by the fire-resistant structure William experimented with using arches made from bricks to construct barrel vaulted ceilings supporting a brick floor. This was a continuous set of arches that looked like a barrel cut lengthwise. These arches sprang from the protected structure at right angles to the beams.

The top floor of Milford Warehouse showing the fireproof structure

The brick arch vaulting and iron columns supporting and beams, each supporting the ceiling vaulting.

Each arch was constructed from a single row of bricks and in the North Mill span the nine feet between each support beam. The space above was infilled with rubble and a level floor laid on top. at right angles to the beams.

William however encountered a problem when trying to make wider arches to span greater distances: as an arch grew wider, the heavier it became and the more likely it was to collapse under its own weight, as well as increasing the lateral force it imposed upon the supporting beams and walls. Traditionally this was resolved by making the arches taller, however this was not an option as the floors they supported needing to be level throughout the building. To resolve this William used hollow pots in the place of bricks to produce an arch that was much lighter and thus had a longer span without the risk of collapse.

Artist impression of the Structure of Belper’s North Mill showing the iron frame supporting the brick arched vaulting that in turn support the floors above. The rubble infill required to create a level floor.

One of the Earthenware ‘Flower Pots’ used in the West Mill. The pots used in other Strutt mills were of a similar design to these.

William Strutt did not invent this method of construction. The Theatre du Paris-Royal, constructed in Paris in 1790 used similar hollow pots in its structure to create wide shallow arches. William Strutt started using a similar system ‘The French Method’ when constructing a mill in Derby in 1792 and he continued using them in subsequent designs for mills. The pots used in the construction of Strutt’s later Belper mills look like flower pots, each approximately 20cm high with straight sides and a single hole in the bottom (the purpose of which unknown).

The vaulting was made by placing a wooden former in the shape of the desired arch, then placing the pots, hole end down on top of this. The spaces between the pots were filled with mortar. Once this had set the wooden former would be removed leaving a self-supporting vaulted ceiling. A lid was placed on each pot to prevent the hollow space inside being filled by the material used to create the floor they supported. The use of these pots was not without problems however, because although light they were not strong enough to support heavy machinery and so could only be used in certain areas of the building.

Mark Higginson selected these pots as his favourite thing from the museum collection saying ‘not everything you see in museums is necessarily a scene stealer. But sometimes the least exciting objects have big and important stories to tell, and these clay pots are a case in point. I’ve always had an interest in architecture, so the activities of the highly inventive William Strutt– designer of Belper’s 1804 North Mill – have inevitably caught my attention. Whilst it is true that Strutt was a pioneer and even during his lifetime was recognised as ‘the father’ of ‘fire proof’ building construction, it is too convenient and too simplistic to give him all the credit for the development of the classic iron-framed late 18th-early 19th textile mill. Indeed, he would probably have eschewed such an honour and anyway history is nearly always more complicated than that. Rather, the evidence is that he was one of a group of scientists, engineers and thinkers who exchanged ideas and then went away and worked them up into solutions to particular problems.  These hollow pots, used both to make a floor ‘fire proof’ and to lighten its weight and thrust, were not Strutt’s invention, but instead were an idea he appears to have got from France, where they had come into vogue during the 1780s, being used in theatres, government buildings and market halls.  Strutt is known to have made enquiries about the method in 1792, at the height of the French Revolution. The latter may well have prevented him from obtaining any plans but this did not defeat him and they featured in both his Derby mill – the construction of which began in that year – and a short time later in the new West Mill at Belper.  So, sometimes genius is not solely generated from within but is more a preparedness to bring together and develop the ideas of others to create something unique and new.  And that is the story these mundane little earthenware vessels tell us.’