SECTION 3.6:CLAYDON PIKE IRON SLAG by Chris Salter

(Note: this report has not been updated since the original in the 1980s)

INTRODUCTION

IRON SLAG FROM CLAYDON PYKE

CATALOGUE OF SLAG AND OTHER INDUSTRIAL DEBRIS


Introduction

There are a number of different processes that can generate slag-like material some of which may not be associated with metal-working. The most common types of non-metallurgical slag-like materials are Fuel Ash Slags (FAS) and Furnace Lining Material (FLM), which could have been produced by any operation that was capable of producing temperatures of 1200oC and above. Normally such temperatures were only produced in the manufacture of pottery or metal artefacts but occasionally a large, well ventilated fire could reach this sort of temperature range. The true slags are the vitreous waste product of a metallurgical process. The commonest slags on archaeological sites are those derived from iron working processes such as smelting or smithing. Copper working residues in the form of crucible slags, that is, the dross floating on the surface of the molten metal which is scrapped off just before the metal is poured into the mould, are well recorded but have not been studied much until recently. The more bulky slags formed during the smelting and refining of copper based alloys are extremely rare in Britain. This may, in part, be because they have not been recognised, as it has been assumed that smelting slags would only be found in those regions with copper ore sources (Salter 1986). Similarly, there now seems to be some evidence that tin was being transported around the country in the form of the oxide ore, to be reduced directly into the copper to form bronze when it was required. The pebbles of tin oxide would be extremely easily missed during the course of any archaeological excavation, and hence its virtual absence from the archaeological record.
The quantity of slag produced for a given quantity of metal smelted varies with the purity of the ore. If a high quality ore was used then only a minimal amount of slag needed to be produced, however, if a poor quality ore was used the volume of slag produced may have been many times that of the metal yielded at the end of the process. In the case of iron, where the quality of the primary ores available vary considerably, this can have important consequences for the types of furnace technology that could be used or at least on the external morphology of the slags formed. If very little slag formed in the bottom of the iron smelting furnace this would not excessively hinder the growth of the metallic iron bloom in the higher parts of the furnace. Therefore, relatively small non slag tapping furnaces could be used. However, if the ore was of a poor quality a large volume of slag would be produced which would quickly choke the furnace unless there was a means of removing the excess slag. Therefore, some form of slag tapping furnace would be required. In general, the hydrothermal and metasomatic iron ores in the highland regions of the south west, Wales and Northern England are of a very much higher quality than the sedimentary ores of lowland England. However, even in lowland England there are local secondary or 'bog' ores, which can be of a very high quality. Even the chalklands which are not usually thought of as an area of rich iron ores can produce pockets of iron sulphides which could be roasted to a very high quality iron. Although these sulphide nodules appear to have been collected on hillforts such as Danebury (Salter 1984) there is no definite evidence that they were being used as an iron ore.
After smelting in the bloomery process, the metallic iron would not be forgeable at low temperatures due to its high non-metallic slag content. The external slag would be removed, then the bloom would be forged at high temperatures (about 1200oC) to expel the majority of the remaining slag. After this, the metal could be forged at normal blacksmithing temperatures (700o1000oC). The bloom fettling and forging operations would produce a fair quantity of slag but the normal forging to shape a piece of iron produces very little slag, although a lot of fine hammer scale would be produced. The recovery of the evidence of iron forging requires specialised soil sampling techniques. The welding of two pieces of iron together requires high temperatures (above 1200oC) and a flux to help remove the iron oxides which form on the surface of the metal. Normally the flux used is fine silica sand, which will react with the iron oxides to form a slag with a composition similar to that of the smelting slag. Some of this slag falls into the hearth to collect to form the characteristic plano-convex (PC) shaped smithing hearth bottom. The slag trapped between the pieces of iron to be welded will be violently expelled when the weld is hammered closed. The slag is expelled in the form of a shower of fine spheres of slag, between 0.1 and 4mm. in diameter, which solidify either while they are in the air or as they hit the ground. In general, the relative abundance of the various ironworking slag types is in descending order: smelting slags, bloom smithing slags, welding slags (hearth bottoms), hammer scale and welding slag spheres. It is, however, unlikely that the last two classes of debris will be recovered in an excavation not specifically carried out to investigate a metallurgical site, due to the very fine scale of the material.
When the smelting slag is in the form of slag tapped out of the furnace it is easily recognised and is a clear indication that smelting has occurred. There are distinct problems with those sites on which only hearth or furnace bottoms are found. McDonnell (1986) suggests that all this material should be regarded as evidence for smithing (forging and welding) only, and for most periods this is probably sound advice. When high quality ores were used in the small non slag tapping smelting furnaces the type of slag formed would be very similar to that produced during the higher temperature forging processes. In forging and welding experiments I have been able to produce small PC slags very similar to those identified as smithing slags. However, these experiments suggest that it would be very difficult to form the larger heavier PC and Furnace Bottom Slags (FBS) by this mechanism. These larger slags can weigh around 600g and contain 50-60 percent iron (360g). Only the larger piece of IA iron exceed this sort of weight. Therefore, if these slags were formed during a single operation, which usually appears to be the case, the blacksmith must have been loosing well over half his metallic iron during smithing. This sort of rate of metal loss would be unacceptable. It seems much more likely that these slags were the result of the smelting of iron in the small non slag tapping furnace.
In the prehistoric period both the small non-slag tapping furnace (bowl furnace) and the slag tapping furnace (shaft or domed) seem to have been in use (Clough 1985). The use of small non-slag tapping furnace must have been limited to where a supply of high quality ore could be obtained. Whereas, the slag tapping furnace would not be so limited, although it would have required a much larger amount of fuel to keep it running. The slag tapping furnace does seem to have been introduced much earlier in the south and east of the country, whereas, the non slag tapping furnace was still in use in the Romano-British period in North Wales (Crew 1985). How the two types of technology changed with period and geographical regions is far from clear. This problem will only be solved by the careful and study of the ironworking debris from a large number of sites from the Iron Age and Romano-British periods, together with experimental reconstruction to produce similar metalworking residues.

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Iron Slag from Claydon Pike

Of the c 32 kg of slag-like material recovered from the site, over 66% came from Phase 3 contexts, and while this is not a huge quantity, it does indicate that some level of industrial activity did take place within the settlement, as would perhaps be appropriate for an extensive agricultural complex. Furthermore around 88% of this material came from contexts in Trench 17 implying that this was very much a localised activity, restricted to the margins of the main settlement area.
There is no evidence for pottery production on site, and the quantity of true slags, which are the vitreous waste product of a metallurgical process, indicates that iron working did take place. The commonest slags on archaeological sites are those derived from iron working processes such as smelting or smithing, and at Claydon Pike it seems that all of the material was derived from smithing. Such iron smithing was probably carried out on a relatively minor scale, possibly for the creation and repair of agricultural tools and structural fittings for the estate.

Catalogue of slag and other industrial debris

Context

description

wt (g)

806/B/1

1 piece of smithing slag

210g

 

1 very fragmented piece of smithing slag

460g

1450/B

Fragmented piece of smithing slag

175g

 

1 piece of smithing slag

35g

 

Unclassified slag

10g

1445/B

3 pieces of slag lining reaction product (SLRP)

33g

 

1 piece of slag cake, smithing

80g

 

1 piece of large PC or FBS but without furnace lining, a piece of granitic? rock adhering smithing slag

425g

 

1 piece of large PC, 15% missing, smithing

575g

1454

3 pieces of broken thick low density PC smithing

530g

 

Complete sample of PC type smithing?

300g

 

Small fragment of material similar to the broken sample

54g

1409/C/1

27 small fragments of various slag types

285g

 

7 nails and 1 fragment of iron sheet

 
 

7 fragments of FLM

29g

 

2 complete pieces of dark coloured FAS

29g

 

Complete rounded low density PC

185g

1409/D/1

Corroded iron

12g

 

Small PC, smithing

27g

 

2 small flows less than 2mm thick

29g

 

Larger PC medium density, smithing?

265g

1409/E/1

PC, smithing?

365g

 

Crushed and fragmented slag

210g

 

Metallic iron

10g

 

4 pieces of furnace lining

30g

 

Small smithing PC

75g

1409/E

3 small to medium sized PC

335g

 

Large high density PC, smithing

700g

 

4 fragments of large high density PC, smithing

625g

 

Large high density PC, smithing

300g

 

2 pieces of low density smithing slag

475g

 

Crushed and severely fragmented slag

475g

1398/B

1 dense PC 50, 70, 38t half piece present, smithing

170g

 

1 PC shaped piece of slag with furnace lining attached 55, 50, 25t, smithing

100g

 

1 fragment of thick PC medium density

40g

 

Several piece of low density porous slag probably smithing

155g

 

1 small piece of low density slag

38g

 

Fragment of larger PC probably smithing

225g

1269/F

1 large piece of FBS slag from immediately below the tuyere zone of the furnace

425g

 

The furnace lining shows that the furnace has been relined on at least one occasion, and that there has been very extensive erosion of the furnace lining. Smithing 1 piece of mixed low density slag probably the result of massive furnace lining erosion

190g

 

7 pieces of SLRP

200g

 

1 flat piece of metallic iron

43g

 

1 heat shattered limestone pebble total weight

1090g

1335/B

Light FAS/FLM

265g

 

9 pieces of medium density unclassifiable slag

250g

 

1 small fragment from a thicker PC type, smithing

40g

 

1 fragment from a larger furnace bottom 25-30 cm diameter 75,65,35t, most likely to be a smithing slag

 
 

1 piece of iron

 
 

1 fragment of an iron blade

 
 

1 piece of curved slag with the definite impression of rod pushed through into the slag

300g

 

1 piece of porous low density slag very firmly welded to chert or flint pebbles-smithing

10g

2357/c/

1small fragment of PC, but it is just possible that there is some partially reduced ore present

18g

2375/K/2

virtually complete section of dense PC slag adhering to furnace lining, curved edge shows that the furnace bottom was roughly 30cm in diameter, strongly magnetic in parts 85,75,50 possible smithing slag

250g

1200

2 pieces of smithing slag

40g

 

1 fragment of smithing slag

119g

2430

2 low density small PC, smithing

125g

 

Small PC with glazed upper surface, smithing

125g

 

2 pieces of thin slag (5-8 mm thick)

19g

 

Irregular flow to PC shape, smithing

79g

 

Fragment of larger PC slag, shape indicates furnace diameter of 25-30 cm, there may be iron adhering to the upper surface

175g

 

piece of conical slag, may have filled the tuyere hole, minimum diameter 22mm,smithing

77g

2430

Fragment of large PC, high density. The fracture appears to have occurred while the slag was hot, smithing

200g

 

Small hollow PC, smithing

37g

 

4 pieces of unidentifiable slag, one may be highly fired crucible or furnace lining

41g

 

Piece of high density slag

15g

 

Iron ore haematite and Goethite showing botryoidal growth form

5g

1440

1 piece of Plano-convex slag, medium to high density, glazed upper surface, smithing

250g

 

1 piece of undefined slag, very rusty surface probably smithing slag

 
 

1 piece of fragmented high density slag probably smithing

300g

1392/2

4 small fragments smithing slag

39g

 

1 fragment of low density PC, smithing

34g

 

1 fragment of medium density PC, smithing..

41g

 

1 fragment of large high density PC, smithing?

48g

1610

1 Piece of FLRP

15g

2491/1

1 block of FBS type slag

570g

2835/C/2

FLRP flow

43g

1447/D

Edge of fragment of PC

110g

1409/E

Large low density PC, smithing?

1690g

 

Massive, high density PC, smithing

1450g

1428/C

Fragment of large PC, 4 smaller fragments and a little FAS formation

84g

1359/B

Large PC with some regions of partially reduced iron ore

600g

1393/B/3

1 fragment of thick PC, possible smithing

 
 

1 lump soil concreted with calcium carbonate or iron

 

1335/B

12 pieces of FLM/FAS highly fired

135g

 

1 piece iron sheet

 
 

5 assorted small pieces slag, larger showing inclusions partially roasted ore, needs further investigation

105g

1273/B/2

Major piece of thick PC broken at edge, smithing

275g

 

A few pieces unidentifiable slag

35g

1388/B

2 pieces low density FAS or FLM

7g

 

1 small piece of medium density slag

11g

 

1 piece of FLM or Flrm

36g

1213/B

Fragment of medium density well formed slag, smithing

60g

607/D

Fragments of thick PC material, smithing

340g

1285/B

SLRP material

6g

2353/1

1 large well formed PC of dense slag 80,80,65t smithing

635g

748/F

1 small piece of light coloured soft slag smithing

50g

2829/B

Fragment of smithing slag

540g

2441/1

Fragment of PC medium density, smithing?

24g

2430

1 piece of dense wedge shaped slag, possibly smithing

260g

2430

Indeterminate shape, with semi-circular channel 12mm diameter running across slag, looks like a rod has been pushed down tuyere to clear slag

65g

1200/2

1 small half PC 65,45,20t, dense well formed slag

120g

 

1 double PC good charcoal impressions 90.75,49

210g

1200/2

Large ovate PC 120,78,40t, virtually complete probably smithing

380g

 

12 fragments of high density slag difficult to classify

390g

 

1 curious shaped piece of FLM, D-shaped but broken

8g

504/1

3 small pieces of high density well formed slag

125g

 

1 piece of high grade non-magnetic iron ore not bog ore but brown streak

76g

1413/B

Roughly circular slag cake, high density and strongly magnetic, smithing

235g

1604

Fragments FLM

80g

 

4 pieces of FLM (could have come from crucibles)

17g

 

1 piece of iron-claystone

5g

 

2 pieces of SLRP

24g

 

9 fragments of smithing slag

230g

563/K/5

1 piece of smithing slag

35g

563/K/2

3 pieces of smithing slag

 
 

1 piece of iron bar

 

1850

1 massive fragment of FLM

37g

649/B/1

3 pieces of smithing slag

38g

732/A/1

1 fragment of smithing slag

9g

2051/2

Fragments and remains of partial fired FLM probably a smithing hearth

940g

533/1

1 piece of smithing slag

24g

534/A/1

3 pieces smithing slag

1g

540/1

Fragments FLM

43g

 

5 pieces smithing slag

46g

 

1 piece iron sheet

 

1730/A/1

1 piece smithing slag

27g

540/A/1

9 pieces FLM and SLRP

45g

 

6 pieces of smithing slag

110g

 

1 arrowhead shaped metallic iron

9g

1202/1

1 piece of smithing slag

38g

559/G/1

1 piece of smithing slag

45g

1465

1 piece corroded iron, possibly nail

 
 

1 piece smithing slag

21g

1409/G

2 pieces smithing slag

188g

 

4 pieces smithing slag with silica inclusions

120g

1409/C

1 piece smithing slag

105g

1335/C

1 piece mixed smithing slag, iron corrosion products and stone

120g

1312/B

1 piece smithing slag

12g

1269/F

Thick piece of FLM and SLRP associated with smithing

200g

2850/A/2

Smithing slag

61g

2823/F

1 piece smithing slag

180g

2819/B

1 fragment smithing slag

34g

2817/A

1 piece smithing slag

83g

1409/D/1

A few flakes hammer scale

 
 

15 pieces broken PC or flat slag

130g

 

3 fragments small PC, smithing

130g

 

Fragments of flat slag

68g

 

High density non-magnetic PC, possibly smithing

250g

 

Small PC smithing

87g

1433/A

5 small pieces smithing slag

14g

1432/A/1

Smithing slag PC

125g

 

Undefinable slag

36g

 

2 small drips of slag

5g

 

1 piece FAS or FLM

4g

2438/A

1 piece small PC, smithing

90g

1429/B/1

3 small pieces slag and a few flakes hammer scale smithing

8g

2390/D/1

2 pieces smithing slag

20g

1367/A

2 pieces of iron

14g

 

5 pieces of FAS/FLM

45g

 

1 slag flow down the side of a furnace, half broken when semi-solid, smithing

53g

1335/F

2 pieces metallic iron

 
 

1 piece unclassifiable slag

42g

1335/D

Fragment of small PC, smithing slag

31g

670/G

2 pieces of strongly magnetic material, probably metallic iron

39g

1294/D

1 piece of PC, smithing

180g

 

6 fragments of smithing slag

75g

 

1 piece of unusual sandy black material, looks like modern core clay after use

40g

1294/E

2 pieces of thin irregular flow shaped slag, highly magnetic, smithing

30g

1294/B

1 small piece smithing slag

30g

667/M

1 piece smithing slag

66g

1228

1 piece smithing slag

32g

1201/J

1 piece smithing slag

7g

 

Completely rusted iron nail and sheet

9g

1296/F

1 piece of FLM

11g

 

1 piece smithing slag

20g

1359/B

2 fragments smithing slag

2g

835/D

1 piece smithing slag

11g

1219

1 small nail, corrosion products

 
 

1 small piece slag

 

2384/C/2

2 small pieces slag

51g

1920

1 piece smithing slag

 

504

1 fragment smithing slag

120g

1200

6 fragments SLRP

83g

 

1 piece SLRP

59g

 

2 pieces slag flow, unclassified

63g

 

2 pieces smithing slag

143g

1200

12 pieces smithing slag

300g

1200

2 pieces smithing slag?

71g

1200

1 piece metallic iron- shallow bowl?

 
 

1 fragment iron nail

 
 

3 pieces iron bar

 
 

1 piece smithing slag

2g

1200

2 pieces smithing slag

18g

2430/2

Crushed slag, smithing

41g

2430/1

3 pieces smithing slag

80g

2398/B/1

1 small fragment PC, smithing

26g

2350/C/1

1 medium sized PC, probably smithing, broken, 60,55,30t

140g

 

1 small PC complete, 60,52,30t, top surface has some silica adhering, could just be copper working

130g

2345/1

1 small piece PC with charcoal marks, high density smithing

150g

1200/2

12 fragments clinkery type material, smithing?

162g

1200/2

4 fragments small PC, typical smithing slag

375g

 

4 fragments FLM

10g

1200/2

Large round PC, with silica rich material on upper surface, copper?

 

1200/2

2 piece metallic iron 25,12t

46g

 

2 pieces low/medium density slag with charcoal impressions

15g

1200

1 small piece hard well formed smithing slag

20g

1200/2

1 piece low density rubbish with silica pebble, smithing

48g

 

1 small piece unidentified slag

5g

 

1 piece FLM

3g

1200/2

12 fragments thick medium/high density PC some included silica, smithing

525g

 

1 piece metallic iron

27g

1200/2

1 small smithing slag PC

125g

1200/2

1 medium sized irregular shaped slag, smithing

125g

1200/2

1 iron nail head

18g

 

1 piece smithing slag

7g

1200/2

1 fragment smithing slag

99g

1200/1

1 piece smithing slag

72g

1200/2

Smithing slag

8g

1200/2

2 pieces of smithing slag

25g

1200/2

2 pieces of smithing slag

27g

1200/21

piece smithing slag

 

522/1

1 piece smithing slag

250g

541/1

1 piece FLM

3g

 

1 piece smithing slag

46g

503

Fragments smithing slag

20g

575

1 piece of smithing slag

 

2713/A/1

Complete PC probably smithing

150g

1392/1

1 small PC smithing slag

53g

1393/A

1 piece of soft low density slag, sample x.1, smithing until matrix identified

50g

 

2 pieces of slightly harder but similar slag

75g

 

1 piece low density, rust spotted smithing slag

175g

 

1 medium/large PC of low density slag, probably smithing

300g

560/1

3 pieces of smithing slag

51g

526/C/1

2 fragments SLRP

5g

533/A/1

SLRP

5g

2161/C

1 piece SLRP

39g

1448

crushed slag

2g

1447/B

3 pieces SLRP

14g

1312/A

2 pieces unclassified slag

9g

2809/C/1

1 piece FLM

29g

2831/B

1 piece FLM

17g

2818/B/2

1 piece FLM

17g

2815/B

1 piece FLM

17g

2806/A

1 piece FLM

17g

1435/A

Large low density PC with inclusions of either silica rich or calcium rich material on the upper surface, possible copper working

475g

1320/F

Disintegrated FLM and soil

60g

667/C

FLM

36g

1337

FLM

 

1305/A

FLM

23g

1201/L

3 pieces of FLM

15g

1286/C

SLRP

18g

1386/A

2 pieces of FLM

16g

1367/K

3 pieces of FLM

13g

1318/A

FLM

14g

1315/A

FLM

76g

1309/G

FLM, poorly fired

13g

1335/A

FLM, poorly fired

19g

1315/B

FLM

2g

1318/A

FLM

1g

1286/K

1 piece unclassifiable slag

48g

 

3 pieces SLRP

 

748/C

11 pieces unclassified slag

135g

501/D/2

FLM

 

687/1

FLM/FAS

80g

693/1

Unclassified slag

5g

504

Unclassified slag

74g

1200

4 pieces unclassified slag

21g

2430/2

Unclassified slag

5g

2430/2

Unclassified slag

4g

1200/2

2 small pieces of FAS

8g

1200/2

2 pieces SLRP

9g

522/1

FLM, possibly crucible

1g

747

SLRP

14g

2430/2

FLM

6g

785

SLRP

4g

1367/G/1

Limonitic/bog iron ore

17g

1367/F

Yellow clayey ironstone

18g

1200/C

Yellow clayey ironstone

7g

1200

1 fragment oxidised marcasite nodule, probably iron ore from chalk

 

890/1

Magnetic iron ore (possibly corroded metallic iron)

17g

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