Ironworking was such an important activity in Britain that without it Roman, medieval and later societies would not have been able to function at the level they did. Without iron York would not have existed. Iron was required for the tools used in agriculture and virtually all craft activities; for the large hooks and chains necessary to lift great weights; for nails, clamps, hinges and other items needed for buildings and their contents; for ships and carts; and for the weapons and armour used by soldiers. The list is virtually endless.

Ironworking was a dirty, noisy activity. The loud hammering, the deep whooshing sound of the bellows, the flame and smell of the forge fire, the dust from smoke and coal were often viewed as anti-social by near neighbours engaged in other trades. Although initially tolerated within towns, in the medieval period there were many attempts to restrict the activities of the smiths to reasonable hours. In the Calendar of the Early Mayors' Court Rolls 1298–9 from London mention is made during the trial of sixteen smiths of an ordinance which stated that smiths should not work at night on account of the unhealthiness of coal and the damage to their neighbours (Adams 1951, 16). As time went on smiths often worked on the edge of towns, though not exclusively on account of the effect their craft had on the environment; in large cities such as London the increasing value of land within the walls led to smiths being forced to relocate to the periphery in order to carry on their craft.


Some 138kg of ironworking slag and related debris was recovered from the excavations. It was not washed on site. Approximately 95% of the total slag (by weight) was visually examined and categorised on the basis of morphology alone. Each category of slag in each context was individually weighed to 2g but in the case of the smithing hearth bottoms each was weighed and measured to obtain its dimensions. Additionally a magnet was passed through the soil within the finds bags to detect any micro-slags such as hammerscale which might be present. Further details of the slag examined and its quantification can be found here [PDF format document].


Ironworking Products

Activities involving iron can take three forms:
  1. the manufacture of iron from ore and fuel in a smelting furnace. The resulting products are slag (waste) and a spongy mass called an unconsolidated bloom which consists of iron with a considerable amount of slag still trapped inside.
  2. primary smithing (hot working by a smith using a hammer) of the bloom on a stringhearth, usually near the smelting furnace, to remove excess slag. The slag from this process will include micro-slags, particularly tiny smithing spheres.
  3. secondary smithing (hot working) of an iron shape by a smith to turn it into a utilitarian object. This will also generate micro-slags: hammerscale flakes from ordinary hot working of a piece of iron, or tiny spheres from high-temperature welding to join two pieces of iron together.
Iron smelting and smithing generate slags, some of which are diagnostic of the process being carried out and others which are not. Slag classified as undiagnostic could have been produced by either smelting or smithing: the process can only be determined in the light of any associated diagnostic evidence from the site. Other slag may be broken up during deposition, redeposition or excavation and, because of its fragmentary nature, it may have to be assigned to the undiagnostic category. Other types of debris may be the result of various kinds of high-temperature activities, including domestic fires, and cannot be taken on their own to indicate ironworking. These include such materials as fired clay, vitrified hearth lining, cinder, and fuel ash slags.

The assemblage from Walmgate represents secondary iron smithing mainly in the form of smithing hearth bottoms. No diagnostic evidence for smelting was present. In some cases slag had to be assigned to the undiagnostic category because it could have been produced by either smelting or smithing, or because it was broken up. The absence of smelting slags, however, suggests these slags were also produced by smithing activity.

Iron slagsOther debris
smithing hearth bottoms75.5kgvitrified hearth lining655g
smithing slag14.2kgferruginous concretion1.8kg
hammerscalenot quantified   clinker27g

Smithing hearth bottoms

Slags diagnostic of iron smithing take two main forms: bulk slags and micro-slags. Of the bulk slags the smithing hearth bottom is the one least likely to be confused with slags produced by smelting. Its characteristic plano-convex-shape (which can sometimes be quite large) was formed as a result of high-temperature reactions between the iron, iron-scale and silica from either a clay furnace lining or the silica flux used by the smith.The predominantly fayalitic (iron silicate) material produced by this reaction dripped down into the hearth base during smithing forming smithing slag which, if not cleared out, developed into the smithing hearth bottom.

Hearth bottoms were usually removed and deposited in the nearest pit or ditch since if left on the floor of the dimly lit smithy they would present a hazard. The proximity to a building of cut features or dumps with amounts of smithing hearth bottoms is often a good indication that the structure may have been a smithy. On the occasions when a number are found inside a building (such as Phase 9.2 context 2217 in Building W) they are usually against a wall or in a corner.

Two hundred and thirty two smithing hearth bottoms were recovered from all phases at Walmgate, their total weight being almost 75.5kg, over half the entire slag assemblage.

Identified Smithing Hearth Bottoms
PhaseWeight (g)

The disposal of smithing hearth bottoms appears to peak in Phase 9.3. Other important phases include 8, 9.2, 9.6, 9.8 and 9.9. The presence of smithing hearth bottoms in earlier phases suggests ironworking, on a small scale, on or close to the site in Phases 1 and 6.3, whilst the later material in Phases 10 to 13 is likely to be residual.

Analysis of the distributions of smithing hearth bottoms shows that before Phase 9.2 they were disposed of in dumps, backfills and levelling deposits rather than being associated with particular buildings. This general pattern continued but smithing hearth bottoms were also incorporated into floor deposits and other structures from Phase 9.2 onwards. The southern room of Building U was a major focus of iron smithing from Phase 9.2 to Phase 9.8. The northern room of Building U was also important, but does not seem to have been used for iron smithing until Phase 9.5, remaining important until Phase 9.8. A hearth (1338) in this part of Building U contained a smithing hearth bottom, probably incorporated into the structure of the hearth when it was built. Building W was also important for iron smithing from Phase 9.2 to Phase 9.4. After Phase 9.4 Building W was not used for the disposal of smithing hearth bottoms. This, however, does not prove that it was no longer used for iron smithing; it may have remained important until it was demolished in Phase 9.8. The function of Building Y within this metalworking building complex is more difficult to decipher, with smithing hearth bottoms only being recovered in dumps, backfills and levelling deposits within the building. Perhaps, as with the non-ferrous metalworking, it was used for storage and finishing from Phase 9.3 to Phase 9.9. Building V also appears to have played a minor role in the iron smithing industry in Phase 9.8, its eastern end being used for metalworking (including iron smithing).

Smithing hearth bottoms
RangeMeanStd. Deviation
weight (g)44–2124325266
length (mm)55–2109423
width (mm)40–2007119
depth (mm)15–904613

Smithing slag

Smithing slag lumps are also fayalitic slags. They are produced by iron oxide, on the surface of the iron, reacting with the silica flux used by the smith to clean the surface of the object being made and inhibit further oxidation of the iron during hot working. It is probable that smithing slag lumps are embryonic hearth bottoms which were never allowed to develop.

Identified Smithing slag
PhaseWeight (g)

As with smithing hearth bottoms, the peak for the disposal of smithing slag was in Phase 9.3, with significant quantities being found in Phases 9.2, 9.6 and 9.8. No smithing slag was found before Phase 9.2 and the material from Phases 11 and 13 is likely to have been redeposited.

The distribution of smithing slag mirrors the pattern of the smithing hearth bottoms. Smithing slag was identified with some key structures including hearth (1563) in the southern room of Building U (Phase 9.4), hearth 1859 in Building W (Phase 9.4) and hearth 1394 at the east end of Building V (Phase 9.8).


Iron smithing also produced micro-slags (generally referred to as hammerscale) of two types: flake and spheroidal. Flake resembles silvery fish scales and is the product of the ordinary hot working and hammering of a piece of iron where fragments of the oxide/silicate skin flake off from the iron and fall to the ground. Spheroidal are small solid droplets of liquid slag expelled from within the iron during the primary smithing of a bloom or the fire welding of two pieces of iron. Hammerscale is not visible to the naked eye when in the soil but is highly diagnostic of smithing activity, often remaining in the area around the anvil and near the hearth when macro-slags have been cleared out of the smithy and dumped elsewhere. Since it is generally highly magnetic, its detection with a magnet during excavation can allow the spatial relationship of the anvil to the hearth to be recorded and can pinpoint the smithing activity more precisely (Mills and McDonnell 1992).

No specific samples were taken to recover hammerscale, but from analysis of the general metalworking samples and the larger slags, the most common form was flake with occasional spheroidal hammerscale. As is to be expected the hammerscale was generally found in lumps from floor surfaces or on slags which were recovered from internal deposits. The following contexts contained hammerscale in some quantity.

2055 (Phase 9.3)A compacted surface (utilising metalworking debris) in the screens passage between Buildings U and V.
1946 (Phase 9.3)A metalworking dump on the western side of Building W.
2008 (Phase 9.5)A metalworking deposit within the northern room of Building U.
1775 (Phase 9.6)A backfill (consisting of metalworking waste) of a pit in the northern room of Building U.
1990 (Phase 9.6)A spread of ash, perhaps associated with metalworking, in the northern room of Building U.
1691 (Phase 9.8)A metalworking deposit within the northern part of Buiding U.

As can be seen, Building U appears to have produced more definitive evidence for smithing taking place in its interior in several phases than any other building. Building W also appears to have been used for smithing. It should be remembered, however, that no samples were taken specifically to recover hammerscale.

Vitrified hearth lining

Hearth lining can vary from highly vitrified hearth lining nearest the tuyère region (the region of highest temperature) to burnt clay (only a little of which was recovered in association with slag). By itself it is not diagnostic of smithing activity, but association of vitrified lining with other diagnostic material provides support for the process.

Cinder is a very porous, highly vitrified material formed at the interface between the alkali fuel ashes and siliceous material of a hearth lining. Again, this material is not diagnostic of smithing activity, but association with other diagnostic material provides further evidence for the process.

Only very small amounts of vitrified hearth lining and cinder were present amongst the slag implying that waist-high hearths with raised fire beds (similar to the type used by blacksmiths until the present day) were used. A survey of this material by the site director suggested that it was recovered from metalworking use deposits and floors from within Building U (in Phases 9.6 and 9.8), Building V (in Phase 9.8), Building W (in Phases 9.2, 9.3 and 9.6) and Building Y (in Phases 9.3 and 9.6). This suggests a shift through time from the southern buildings (Buildings W and Y) to the northern, street frontage buildings (Buildings U and V) for the deposition of these materials. This could reflect a change in the use of buildings for metalworking activities through time, or a change in disposal practices.

Other fragments of fired clay furnace structure which were recovered from the excavations and analysed by Catherine Mortimer (see non-ferrous metalworking) came from the following contexts:

2418 (Phase 6.3)Dump deposit on the eastern side of sunken work area.
2145 (Phase 8)Internal levelling deposit within Building R.
1946 (Phase 9.3)A metalworking dump on the western side of Building W.
1859 (Phase 9.4)Floor level edge-set tile hearth in Building W.
1617 (Phase 9.6)Backfill of a post-hole within Building W.
1783 (Phase 9.6)Floor material within Building W.
2340 (Phase 9.6)Infill material from the southern end of a large furnace construction cut in the southern room of Building U.

Building W seems to have been an important building and probably housed a furnace for metalworking. Building U may also have housed a furnace. The earlier material from Phases 6.3 and 8 may have been imported from beyond the excavation limits.

Ferruginous concretion

Ferruginous concretions are made up of a redeposition of iron hydroxides (rather like iron panning), enhanced by surrounding archaeological deposits, particularly if there is iron-rich waste present as a result of ironworking. A small amount was recovered at Walmgate, its presence is almost certainly due to the large amount of ironworking waste in some deposits.

Offcuts and tools relating to ironworking

Levels in Phase 1 produced SF01581, an iron strip which has been made from three smaller iron strips welded together; it probably represents smith's waste. Phase 1 also produced some hammerscale waste.

Phases 2–4 produced no ironworking fragments.

In Phase 5, a huge iron bar (SF01269) and a plate fragment (SF01383) appear to represent smithing waste. A metalworker’s punch, 127mm long, with a wedge-shaped tip (SF01312), was found in the same phase; it could also have been used by an ironsmith.

Phase 6.2 produced a possible iron punch (SF01288) and an iron strip (SF01328), both from dumps covering the area where Building M had formerly stood; these could have originated from Phase 6.1 metalworking deposits, however. The only artefact that might derive from ferrous metalworking during Phase 6.3 is an iron strip (SF01347).

From Phase 8, an iron plate fragment (SF01070), iron strips (SF01053, SF01067) representing ironsmithing debris, and a possible smith’s punch, 112mm long, with a rounded cross-section and a pointed tip (SF01182), were recovered.

An offcut (SF01323) and two plates (SF00830, SF01317) were recovered from Phase 9.2, and an iron plate fragment (SF01369) and an iron strip (SF01203) from Phase 9.3. In Phase 9.6, floors within Building W produced an iron tanged punch (SF00900) which would originally have had a wooden handle. It may have been used for working metal or for other crafts such as leatherwork or woodwork.

Building U seems to have continued to be used as a blacksmith’s shop during Phase 9.8. Smithing waste found here included sheet fragments SF00493, SF00990, and also SF00751, a product of metalworking containing the remains of sheet, bar or plate fragments. Other ironworking waste such as sheet fragments (SF00535, SF00915, SF01296) was found in other levels. The fragmentary blade of a possible iron sickle (SF00737) was found in a deposit of probable smithing waste, and may have been scrap destined for recycling.


The following table shows the total weight of iron slags recovered from the Walmgate excavations by phase.

All analysed iron slag
PhaseWeight (g)

As can be seen the phases with most iron slag are (in order) 9.3, 9.9, 9.8, 9.2 and 9.6 . The evidence also suggests limited ironworking in the vicinity of the excavation area in Phases 1, 6.1, 6.2 and 6.3, increasing in scale in Phase 8. Structurally, the only building to be associated with these activities during these early phases was Building N in Phase 6.1. Iron slag and other related debris from Phases 10 to 13 is thought to be residual.

The ironworking material found at Walmgate is typical for an urban smithy of moderate size. Before Phase 9.2 ironworking waste was disposed of in dumps, backfills and levelling deposits rather than being associated with particular buildings. From Phase 9.2 onwards, the evidence suggests that Buildings U and W were the main focus of the ironworking activity, with hammerscale evidence suggesting that the northern room of Building U was used as an iron smithy in Phases 9.3, 9.5, 9.6 and 9.8 (though the exact location of the furnace within this room could not be pinpointed). The southern room of Building U also appears to have been important from Phase 9.2 to Phase 9.8. Building W played an important role in ironworking from Phase 9.2 to Phase 9.6, and ironworking may have continued within the building until its demolition in Phase 9.8. Ironworking may also have been carried out within Building Y in Phases 9.3 and 9.6, but the exact nature of this was difficult to ascertain. Ironworking, on a limited scale, also appears to have been carried out within Building V in Phase 9.8. In Phase 9.9, the area to the east of Building Y was the main focus for dumping iron smithing waste (perhaps from Building U).

Click to enlarge
Medieval iron smithing

Click to enlarge
Buildings U, W and Y under excavation

Click to enlarge
Possible corner furnace (1837) in Building V, Phase 9.8

Click to enlarge
Hearth 1563 in Building U

Click to enlarge
Floors and hearth 1859 in Building W

Click to enlarge
X-ray of tanged punch SF00900
© Copyright York Archaeological Trust 2003