They are not setts or cobbles. Setts are normally rectangular, made out of quarried rock, and cobbles are usually smaller and more rounded. Our paving bricks are sometimes called block or stable paviours, as in the past they were used in stables because of their qualities. They were highly resistant to erosion by cartwheels, and also to the uric acid produced by horses, leading to their use in stables.

They appear in South Bank and many other suburban character areas of York, such as Burton Stone Lane, Leeman Road, Haxby Road, the Groves, Holgate, Acomb and Heworth. Although some people think they are unique to York, they can be found in other towns and cities in NorthEast England, such as Saltburn, Darlington, Newcastle-upon-Tyne, Thirsk and Whitby, as well as further afield, for example Stepney in London.

They are actually called scoria bricks or blocks, because they were made from iron slag. Scoria is an ancient Greek word meaning ‘refuse’ or ‘dung’, applied to slag from molten metal, and used by the Romans for basaltic lava, ejected as fragments from a volcano. Their origins lie in the blast furnaces in Cleveland in North East England. This was a successful industry, in 1872 nearly two million tons of pig iron was being produced there, from ore deposits in thick seams in rock in Cleveland and North Yorkshire. When pig iron was produced, it generated millions of tons of slag waste at the bottom of the blast furnaces, a real problem for the ironmasters, as it was expensive to remove. There were many attempts to use molten slag to make bricks, either for building or for paving. But getting the slag speedily into the right place, shaping it into uniform bricks and cooling it slowly was full of difficulties.

It was a Darlington man, Joseph Woodward, who found the solution in 1869. He used a process that tapped the molten slag from the bottom of the blast furnaces and transported it in bogies or trucks along a railway track to a revolving table, his patented invention. The slag was poured into moulds on the table and after two minutes it was still red hot, but set. Then at the vital moment, the turning motion of the table and a lifting latch tipped the brick out of the mould, it was then transferred to and placed in a kiln, as hot as the melting point of slag. Bricks were housed here for three days and allowed to cool very slowly, in order to remove internal stresses and make them easier to work. By 1872 a joint stock company, the Tees Scoriae Brick Co. (TSBC), had been formed to work his process.

The process produced scoria bricks, which come in various shapes. They were very hard to break, withstanding a crushing force four or five times stronger than a common brick. They did have some memorable characteristics. They were liable to explode shockingly if heated around a fire, and they smelt of sulphur/rotten eggs if split. The alternative to using these bricks on 19th century roads and back alleys was whinstone setts, which had replaced cobbles, but scoria bricks were cheaper, and especially useful for back alleys and gutters.

Scoria bricks were used on main roads as well as back streets, for example in the approaches to the new railway station at York, which opened in 1877 (recently uncovered when the Queen Street Bridge was removed).

Sadly in York back lanes have been excavated for utilities and street lighting, and in many cases the scoria bricks have been patched with tarmac. They are now listed by the York Civic Trust as ‘heritage at risk’. Clements Hall Local History Group has been mapping their use across the city (www.clementshallhistorygroup.org.uk/blog/scoria-bricks-do-you-have-them-in-your-neighbourhood).

They have achieved a cultural impact however, both in York and in the northeast generally. In 2015 the first floor extension of York Art Gallery included more than 300 double hexagon shaped tiles fitted to the exterior of the Centre of Ceramic Art(CoCA). Scoria bricks can also be found at the Beamish Open Air Museum.