The first tin blowing mills may have existed on Dartmoor as early as the 14th century. Before then the limitations of the technology had made it necessary for tin to be smelted twice. After a first smelting which may well have taken place at or near the site of the tinwork, the roughly smelted tin was taken to a Stannary town for a second smelting to produce a more refined product. A tax payable at a rate of 30d. (12½p. in today's money) per thousandweight on the first smelt was supplemented in 1198 by an additional 13s.4d. (67p.) per thousandweight at the second smelting. However, probably as a result of improved technology which allowed smelting to be achieved in a single process, both taxes were replaced by a single tax of 1s.6¾d. (8p.) per hundredweight on the finished tin in 1303. It is likely that the blowing mill was developed during this period and was an integral part of this transition.
Although documentation confirms the existence of tin blowing mills in Cornwall as early as 1402, on Dartmoor it is not until the early 16th century that the first documented blowing mills are mentioned. At Dartmeet for example a 'blowyng mill' had recently been built in 1514. However, this lack of earlier references is more likely to be due to poor survival of documents and it is highly probable that blowing mills had been at work on Dartmoor for quite some time before this. We have many more references to blowing mills from the 16th and 17th century, particularly the first half of the 16th when the industry was enjoying something of a mini-boom. Nevertheless, by 1730 only two blowing mills were operating in Devon at Sheepstor and at Plympton. These may reflect a short period of prosperity for the industry which is recorded in the late 17th and early 18th century and were probably among the last smelting mills on Dartmoor to use the blast furnace.
The most commonly occurring field evidence to confirm that a tin mill was used for smelting or 'blowing' is the mouldstone (Photo No.1). These are large pieces of flat-topped granite with a rectangular hollow recessed in the upper surface, into which molten tin was poured to be cast into ingots. The mouldstones vary in size and shape but needed to be fairly substantial to absorb the high temperatures involved in the casting. There is similar variety in the dimensions and form of the hollows which may be as large as 28cm x 39cm as at Upper Merrivale (Photo No.2) or as small as that at Longstone which measures 18cm x 18cm. The bottom surface of the hollows are often flat but can sometimes be slightly concave, which would in turn have cast a slightly convex ingot. Some mouldstones have a small groove cut into the upper lip on the narrow side of the hollow. This feature is particularly clear on the mouldstone at Upper Merrivale. R.H. Worth believed that a stick would have been rested in this groove before the molten tin was poured, leaving a hole through the finished ingot by which to lever it out and as a means of tying it to a packhorse for transportation. Exactly how the stick would have borne the temperature of molten tin at approximately 1200° C is uncertain but presumably the metal would have set sufficiently before the stick was burned. The only properly recorded find of a tin ingot from Dartmoor, did indeed have a hole through it in the position we would expect if this idea had been practiced.
Several mouldstones have one, or sometimes two, additional, smaller rectangular hollows on their upper surface. They measure on average 9cm x 5cm x 1-2cm deep. These are traditionally believed to be sample moulds or 'trying' moulds for testing the purity of tin as part of the assay process. Another possibility is that tinners would cast these small, easily concealed ingots to be sold illicitly without the knowledge of tax collectors.
All blowing mills would have housed a furnace (Photo No.3) but only a very few have survived on Dartmoor as visible field evidence. Unfortunately we have equally few descriptions of furnaces or smelting technique by contemporary writers in either Devon or Cornwall, especially for the earlier period. Thomas Beare in 1586 alluded briefly to some of the smelting practices he observed in Cornwall, mentioning some details, including the use of bellows and probably machinery to power them. Also, in 1778, another Cornishman called William Pryce explained the principals of tin smelting in a forced draught furnace called a 'castle' in his treatise on Cornish mining. Although quoted several times before it is worth reproducing Pryce's description here as it is so apposite:
The fire place or castle, is about six feet perpendicular, two feet wide in the top part each way, and about 14 inches in the bottom, all made of moorstone and clay, well cemented and clamped together. The pipe or nose of each bellows is fixed ten inches high from the bottom of the castle, in a large piece of wrought iron, called the Hearth-eye. The tin and charcoal are laid in the castle, stratum super stratum, in such quantities as are thought proper.
Although written some 200 years after the boom of the tin industry in Devon and probably after forced draught furnaces had mostly ceased to operate on Dartmoor, comparisons between the furnace described and surviving furnaces in Dartmoor blowing mills may certainly be made.
The field evidence for a furnace comprises two upright, roughly rectangular granite slabs, set into the ground and spaced approximately 0.5m to 0.8m apart (photo No.3). Such stones survive in place at Lowe and Upper Merrivale, Avon Dam and Blacksmith's Shop. At Lower and Upper Merrivale a further slab at the back completes what resembles an open fronted hearth. Other examples with back slabs in situ are lacking and we cannot assume that they were necessarily a permanent component, although the probable furnace at Week Ford also has a back slab in place. Surviving furnaces are always sited within the floor area of the mill, so that bellows could be placed behind to blow into the rear of the hearth, and are never built into the walls of the structure, as would a fireplace. Even at these surviving furnaces it is clear that the remains only represent the basic shell and it is likely that the upper section of the structure was built in a less enduring manner and has since collapsed. Although some mills have what appears to be a furnace built into a wall as at Black Tor Falls right mill (Photo No.4), these are more likely to have served other purposes and were not used for smelting.
In the furnace, prepared cassiterite and peat charcoal which acted as a fuel and a flux were packed in, layer upon layer, as per Pryce's description. Thomas Beare, when describing a smelt of tin, seems to suggest that the furnace was heated before the tin was introduced. Once alight a forced draught was created using bellows powered by the waterwheel. A single smelt was called a 'tide' and according to Thomas Beare had a duration of approximately 12 hours, during which time different types of cassiterite (usually referred to as black tin at the pre-smelting stage) were introduced in a set order, beginning with streamwork tin, followed by 'moor' tin (from lodes), then small 'myn'' tin, then the waste.
At Lower Merrivale mill, located within he base of the furnace is a flat-topped slab with a shallow trough, measuring 30-35cm wide by approximately 8cm deep, cut into the upper surface. This is usually referred to as a floatstone in which molten tin would accumulate during the tide, then to be ladled into the mould. The 'flote' was also mentioned by Beare which certainly confirms it as a component of the furnace. The floatstone at Lower Merrivale and that from Longstone, Sheepstor are still the only surviving examples, despite the excavations at Upper Merrivale where a furnace has been uncovered but no floatstone. When tin is smelted in a furnace, the process produces a quantity of a black glassy substance (Photo No.5) called slag, which is a waste product of the reduced ore. It often contained some unrecovered cassiterite and it was well worth the while of the smelters to collect and crush it for re-smelting. Samples of slag may often be found lying on the ground surface near tin mills. The cassiterite shows up as small pale flecks within the black matrix of the slag.