Jesse Shirley’s Bone and Flint Mill

See mill in operation

During the first part of the 18th century the beneficial use of ground flint and bone was discovered. Flint (i.e. silica, up to 50% of the total) can be added to clay to produce earthenware products, it gives the ware strength, whiteness and prevents shrinkage during firing to make a hard cream product. The problem with grinding flint using the technology of the day was that it produced clouds of dust; the workers quickly died of “Potters Rot” (silicosis of the lungs) and would not undertake the work. Consequently, the wet pan grinding method was developed to reduce harmful dust; this is illustrated at Jesse Shirley’s Bone and Flint Mill.

Jesse Shirleys Bone and Flint Mill 2_renamed_27316

Cattle bones were found to be the most suitable for adding to clay (again up to 50% of the total) to produce bone china. It is the bone which gives the ware its characteristic translucent quality, it is whiter than other ware and its high strength allows it to be finer. About 1747 it was discovered that Cornish stone (a partly weathered granite) mixed with china clay would form a porcelain body. Small quantities of Cornish stone were also processed on site. The two processes for bone and flint were similar and the ground products revolutionised the ceramics industry; existing water powered corn mills were converted and new water mills were built. Wind could not supply the continuous high power required, but steam power was an obvious application as steam engines became more powerful and reliable. Thus building of the steam powered Jesse Shirley’s Mill was commenced in 1856 at the Junction of the Trent and Mersey Canal and the Caldon Canal as canals offered cheap transport of these heavy raw materials.

Jesse Shirley was born in 1819. In 1834 he was employJESSE SHIRLEY SITE 1857 001ed by his step-father John Bourne at his firm of Bourne and Hudson Bone Works, originally as a writing clerk. When John Bourne died in 1852 he left the business to brothers Joseph and Jesse Shirley. It was the latter who had the present Mill built in 1856/1857.

 

The picture shows the mill when it was built in 1857. The buildings in the foreground, chimney and the machinery housed remain largely unaltered.

The road access to the site was poor but the location was chosen because of the proximity to the canals and the availability of a wharf so providing easy access to many local potteries and to all parts of the country.

Bone, usually from cattle would originally have been sourced locally but as demand increased it was sourced from various parts of the country and latterly from overseas. Flint was from the south and east coasts of England and near continent.

Calcining Kiln

The kiln was used to calcine (roast) flints and bone to approximately 1000 degrees centigrade to change their nature and make them suitable for grinding to a fine powder.

CALCINING KILN_20110722_0445_renamed_11188

Flints are an unlikely raw material for pottery as they are hard and black in their natural state. If they are calcined above 1000 degrees centigrade crystalline water is driven off to leave a softer, lighter and whiter product.

The calcining kiln consists of two chambers with a hovel built above them to create a draught to aid combustion. Filling the kiln was a very skilled job requiring layering of fuel and either bone or flint. Flints would be built up in layers with slack (small pieces) coal using approximately 1 hundredweight (51Kg) of coal per 1 ton (1.02 tonnes) of flint. It would be allowed to combust for 8 to 16 hours (depending on the fuel and climatic conditions) and then left to cool before being withdrawn through draw holes at the bottom.  Production of ground flint ceased at the Mill in the 1930s

Bone was treated in a similar way after first being boiled to remove tissue, this would produce glue, a saleable by-product.  Wood was used as the fuel as bone is more combustible and is prone to contamination from iron pyrites in coal.  Calcined bone is softer and whiter than in its natural state.

 

 

Crusher Room

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This is now entered down steps, but originally the floor was level with the canal wharf outside. Mining subsidence has lowered the whole area by about 6 feet (2 metres) and the canal wharf has had to be raised to maintain the canal level – see the blocked up lower part of the windows in the Gear Room. Here are the two draw holes for the kiln.

The crusher was installed in the 1930s after the calcining kiln seen today was decommissioned. It is of the oscillating jaw type, belt driven by a small horizontal steam engine of unknown manufacture and date and was used to crush oversize flints and Cornish stone.

 

Gear Room

Power is transmitted via the flywheel axle of the engine through a ratchet which enables the engine to be barred backwards by hand if it has stopped on a dead centre without moving any of the machinery backwards. Rim gears transmit the power to long line shafts which run the length of the room, one shaft is currently driven. Along each line shaft are five bevel gears which drive vertical shafts taking the power to the pan room above.

Mill GEAR ROOM_20110722_0453_renamed_25161

To separate coagulated particles in the ground mix it was run off from the pans in the upper room through wooden launders to one of three wash tubs where rotating paddles stir the mix and ‘blunge’ it through the vertical bars of the paddles.

The mix was then run into one of two rectangular settling arks. Much larger arks holding 25 tons (25.4 tonnes) of liquid were beyond the gear room wall and have been lost during development of the site. As the solid particles settled wooden bungs in a vertical plank were knocked out to run off the clear water. This left a thickened slop which could be put into barrels and sold or pumped to drying beds (which have been lost) where water was evaporated off and the solid product sold as blocks. Above the arks is a rare ‘Pulsometer’ steam pump. It has two chambers, steam pressure empties one side whilst the vacuum from condensed steam draws fresh liquid into the other side. A steel ball then rocks over a knife edge to allow the process to be repeated on the other side.

Pan Room

Material to be ground was hoisted through a hatchway in the floor from the Gear Room below. The slack chain hoist was driven from the extended vertical shaft of the small end pan. The material was tipped into one of ten pans and water was added. The pan floor is composed of chert blocks with the gaps filled with pitcher’ (broken biscuit ware). Power from the floor below rotates sweep arms which push large chert blocks or ‘runners’ around the pan. The material is pushed and tumbled around the pan and is ground in the process.

Mill PAN ROOM_20110722_0456_renamed_2770

The larger diameter pans contain runners of up to 1 ton (1.02 tonnes) in weight and a hand crane at one end of the room was used to lift these into the pans. After about 8 hours for flint, less for bone, the material was ground and could be run out of the pan to the floor below for further processing.

Boiler Room

Steam is generated in a ‘Cornish’ boiler built at the nearby Cliffe Vale Boiler Works in 1903. It is hand fired with coal and contains about 2,500 gallons (11,000 litres) of water,  indicated in two water gauge frames with the steam pressure by a Bourdon gauge. The dead weight safety valve releases pressure at 60 pounds per square inch (4.2 kg force per sq. cm), although the boiler is operated at less than half that pressure. It is important to maintain the water level in the boiler and new water from a large iron tank in the roof was originally pumped in, against boiler pressure, by two Weir steam pumps. Examples can be seen against the side wall. Today electric pumps are used.

Steam is taken off from the top of the boiler through a large diameter pipe and taken to the engine room next door

Boiler Room

Steam is generated in a ‘Cornish’ boiler built at the nearby Cliffe Vale Boiler Works in 1903. It is hand fired with coal and contains about 2,500 gallons (11,000 litres) of water,  indicated in two water gauge frames with the steam pressure by a Bourdon gauge.

BOILER 1903_renamed_16792

The dead weight safety valve releases pressure at 60 pounds per square inch (4.2 kg force per sq. cm), although the boiler is operated at less than half that pressure. It is important to maintain the water level in the boiler and new water from a large iron tank in the roof was originally pumped in, against boiler pressure, by two Weir steam pumps. Examples can be seen against the side wall. Today electric pumps are used.

Steam is taken off from the top of the boiler through a large diameter pipe and taken to the engine room next door

 

 

 

Engine Room

“Princess” is a double acting condensing rotative beam engine to the design of James Watt. She was purchased second hand and installed when the mill was built in 1856/7. Her previous history is unknown but she is thought to have been built by Bateman and Sherratt of Salford, Manchester in the 1820s who were rivals and competitors of Boulton and Watt.

Etruria Industrial Museum Beam Engine_renamed_21367

Steam enters the single large vertical cylinder through a main steam valve and is directed to one end of the cylinder via valves which are operated by a rocking shaft beneath the floor which is, in turn, operated by an eccentric from the flywheel axle. Exhaust steam is converted to water in a condenser (cooled by water from the canal) located beneath the floor. This creates a vacuum in the condenser which is opened to the other side of the cylinder thus the pressure difference between steam pressure on one side and vacuum on the other moves a piston within the cylinder. The valves change over and the process is repeated on the other sides of the piston. The two gauges on the wall show the strength of the vacuum and steam pressure.

For the engine to be double acting (pull and push) the beam must have a solid connection to the rod which emerges from the cylinder head through a ‘metallic’ gland packing. The piston rod must travel in a vertical straight line but the end of the overhead beam which transmits the power transcribes an arc as it rocks. This would try to pull the rod backwards and forwards which it cannot be allowed to do. James Watt overcame this problem with his great invention – parallel motion; a trapezoidal connection. At the other end of the beam is the sweep rod which connects via a crank to the axle which carries a flywheel of 20 feet (6.1 metres) in diameter and 10 tons (10.16 tonnes) in weight. The flywheel’s momentum carries the engine over top dead centre and bottom dead centre when the piston is at the end of its stroke and is not providing any power. The axle extends through the wall into the gear room.

Restoration

In June 1978 Jim Kelly, then the Keeper of Social History at Stoke-on-Trent Museum, called for volunteers to restore the Mill and machinery.

Volunteers Receiving an English Heritage Angel Award 2012

It had been in continuous use from 1857 until work stopped when modern machinery on site had been commissioned to replace it in 1972. During that time it had seen little alteration, it had become neglected during its latter days, but was essentially as it had been built well over a century before. The first working party took place on 22nd October 1978 and voluntary work has continued to the present day, some of the original volunteers are still on the team over 34 years later.

The site’s historical significance was recognised in 1975 with it being designated a Scheduled Ancient Monument with the buildings being grade 2* listed. The site was officially opened to the public by Fred Dibnah on 6th April, 1991. It continues to be maintained and operated by volunteers.