Shipshape 5

 

How to build an Anglo-Saxon ship

5. Keel & Frames, Fixings

Paul Constantine

 

August – September 2021

Technical people

After delays caused by Covid lockdowns the project began to move ahead guided by Tim Kirk (right) assisted by Laurie, Alec and the team. Some work had been done prior to this point and all the keel support structure was in place.

 

Keel Support

The keel is at the bottom of the ship, but it is not laid upon the ground. It has to lifted to give access for working on the underside of the hull from the outside. Another factor is that the keel is not straight, but is raised at the ends, so the keel supports must be lower in the middle. All supporting/guiding formwork was painted black (see Tim with brush) to identify it as not being part of the ship. It was also hoped that when official videos are made of the build, technology could omit the black areas, showing only the ship. The supports can be seen in the previous Shipshape 4 section.

 

The Keel

Mention has been made already of the thickness of the keel due to crushing in the ground and the difficulty of defining it precisely. The keel is slightly dished along the inner surface and an additional 1in thickness allowed for its compression over that detailed in Volume 1. A picture on the Home page of this site shows the keel being trimmed by Alec and just behind it the black keel supports can be seen.

It was hoped that the keel would easily sag down in the middle to sit on its supports, but it proved to be very stiff and needed pulling down (left) to encourage it to take the amount of rocker needed.

 

Underlout

Shipshape 4 explained how the ‘stem’ was not a single timber, but was made up from two pieces. The lower piece is known as the underlout and the outer, forward piece is usually referred to as the stem/stern. The rough timbers were temporarily bolted together whilst they were being shaped.

 

Shaping curved timbers

When the selected timbers arrive in the shed they may be round and with the bark still on. To make them approximately rectangular a series of ‘valleys’ are axed into the sides. The bottom of the valleys indicates roughly where the squared sides of the timber will be. They give a guide to the removal of the ‘hills’ between the valleys. Damian is working a piece with the ‘valley cuts’ marked in chalk  Certain timbers are laser scanned on arrival, so that progress can be checked and comparisons made about their shape, shrinkage etc. later in the build process.

 

Frames

Once the keel is positioned it is possible to put the black guide frames onto it.for the middle section of the ship. In the picture some longitudinal braces are still in unpainted timber.

 

Components

Trenails

Visitors to the Longshed can see various examples of investigations that have been made to try to see how effective components and systems are. Here are some pictures of trenails and there are other investigations such as the different rivet arrangements.

 

Rivets

The rivets were vitally important in the original excavation. From them it was possible to calculate the thickesses of the timber at different locations and from the angle of the roves, to see the internal shape of the craft. There are many questions to be answered relating to rivets their roves and caulking. Sizes, shapes and materials. Some experiments were carried out using copper, as rivets/roves are commercially available. Attaching planks to each other could be done easily, irrespective of the material in the rivet.

 

Roman iron

The ship was built in a post-Roman Britain and although it is Scandinavian in form it is wise to never forget this. There is much material relating to reconstructed Viking craft and so, in the absence of pure Anglo-Saxon information, there is a strong attraction toward referencing this material and back-dating it to apply to this ship. Looking at iron working from other areas that fell within the Roman sphere might prove useful.

 

Iron was being produced in Britain for hundreds of years before the arrival of the Romans. It is well known that they used a great deal of iron. They would mine iron, usually as open-cast and two areas of special activity in the south of England were The Weald and the Forest of Dean. These centres of iron working were determined mainly by the availability of timber for the charcoal. We know from the amount of slag produced that many thousands of tons of iron would have been made at just a single site. It is thought that Britain exported iron to other areas of the Roman Empire. The Roman navy fleet Classis Britannica had its own sites for iron production near Hastings on the south coast for 150 years.

 

Mixing Fixings

On a visit to the Newport ship, Dr Toby Jones and Bob Evans drew my attention to the Barland’s Farm boat, a Romano- Celtic craft probably about 11.4m x 3m, excavated 1993 in South Wales. It used sawn oak, had a mast step and was for moving local agricultural goods. The timber showed that it was constructed about 300AD, so it was roughly contemporary with the Nydam ship. The method for plank fastenings was a clever blend of trenail with clenched iron nails where a trenail was inserted into the rib and the metal plank-securing nail driven through it. The nail shank was then bent over and driven into the internal timber. Whilst there is no suggestion here that this method was used in this Anglo-Saxon ship, it is of great interest, as it demonstrates the malleable quality of the Roman iron nail, which may be useful in today’s rivets. The craft is in storage in Newport and the fixings might possibly be analysed for composition?

Scandinavian Iron

It is near impossible to find comprehensive information about the composition of Anglo-Saxon rivets, but it is possible to look at information relating to Viking craft reconstructions – always bearing in mind that the original craft themselves date from 400 years after the Anglo-Saxon ship whose construction could have been influenced by Roman traditions.

 

Bog Iron.

The composition of the iron used in the Anglo-Saxon ship has been difficult to quantify precisely, as it was heavily oxidised and preservation was not good. It is well known that Scandinavian cultures made extensive use of Bog Iron. A simple explanation of this material would be that ground water passing over iron deposits will absorb some of that iron. If a natural stream has a distinctive reddish colour, it is an indicator that iron is present. Plants growing in the water produce oxygen. The iron in the water reacts with oxygen leading to chemical changes which allow consolidation of the iron into large lumps in marshy areas. This boggy iron is relatively easy to find and easier to smelt than solid iron ore when using limited technology. It just needs a ready supply of timber for charcoal to get the temperature required and clay to build a very simple (bloomery) furnace to amalgamate the iron (see right). With simple smelting of this nature the iron never reaches a liquid state as that requires a higher temperature. The iron becomes more ‘plastic’. It has to be heated and worked several times to further consolidate it and remove the impurities called slag. It has been widely used by many cultures. Pure iron is quite soft as the atoms can slide past each other. Mixing in very small amounts of carbon (charcoal) hardens the iron as it locks the atoms. Smelting without bringing the iron to a molten state avoided the carbon mixing into the iron and the iron remained ductile. We cannot use today's mild steel for the rivets as it contains carbon and is too hard.                                                                     Pic. Eric Dennis

One great advantage of bog iron is that - the stream, the marsh/bog continues to produce the iron, so that after a number of years it can be collected again from the same source. 18kg of bog iron would produce about 3kg of usable iron.

 

Scandinavian experience

The half-dozen craft discovered at Roskilde are referred to as the Skuldelev ships. The Skuldelev 3 reconstruction launched 1984 after a 2year build, was an oak 14m trading vessel, which was used at Roskilde for 32 years. Very good records were kept relating to the life of this reconstruction and they are available on the internet in a document called Roar Ege - The Lifecycle of a reconstructed Viking Ship by Tríona Sørensen and Martin Rodevad Dael. EXARC Journal Issue 2020/2.

 

This extract deals with the subject of the rivets. This sailing ship is different from the rowed Anglo-Saxon ship - BUT - The problems of corrosion are the same.

Extract

‘The other significant problem our reconstructions face is the rate at which the iron rivets corrode. They expand as they rust and this leads to cracks occurring in the planking material around the rivets. The iron fastenings used in the construction of Roar Ege are perhaps the main element that deviates from the project’s ambitions about using the same materials as would have been used in the Viking Age. In the early 1980’s, very little archaeometallurgical analysis of ship’s fastenings had been carried out.

Siemens-Martin steel, produced using the open-hearth method in Daval, France, was chosen for the production of rivets and roves (Andersen, et al., 1997, p.44). Siemens-Martin steel is a very homogenous material, and with the benefit of hindsight, it could be argued, that it bears very little resemblance to the heterogenous bog iron the majority of Viking Age ship’s nails were produced from (Lyngstrøm, 2008, p.9). However, at the time, the team felt that Viking Age boatbuilders would have sought the best-quality material they could source for fastening their ships and that the purity of Siemens-Martin steel, coupled with its low carbon-content, would make it a good match for this hypothetical material (Andersen, et al., 1997, p.44; Buchwald, 2005).

Our experiences with the rapid rate of corrosion and rust of the rivets in Roar Ege – and with the other subsequent Skuldelev reconstructions – would seem to suggest otherwise. Rivets tend to need to be replaced after ca. 15 years (rivets below the waterline degrade at a faster rate than those above) and the majority of our reconstructions have seen several phases of rivet replacement through the years. While rivets that have been removed from ships are a common enough find in the archaeological record - especially at ship-breaking sites such as Fribrødre Å here in Denmark - there is limited evidence for a double-imprint of the accompanying rove on surviving ship’s components (Skamby Madsen and Klassen, 2010). This is something you would expect to see on the archaeological timbers if the original ships had also been subject to repairs as extensive as our reconstructions.

In recent years, the theory that the relatively higher phosphor content in the bog ores used to produce Viking Age iron may have made the material more corrosion-resistant than modern day iron has gained traction (Buchwald, 2005, p.173). This is an issue that the Museum will be setting an increasing focus on in the future, as it is of significance both in terms of furthering our understanding of Viking Age metallurgy and potentially prolonging the lifespan of our full-scale reconstructions.’

A range of different rivet forms - square, round etc. in the Longshed. It is thought that a shank dimension of 3/8” (9mm) will be used.

A Conclusion.

There are maps showing where bloomery-furnace sites have been identified in the UK and they look like the country is covered in spots. Bloomeries were increasingly used everywhere from about 3000BC onwards until the introduction of blast furnace technology from about 1500s AD. It is a generalisation, but almost every hamlet, village, small town, city had its own bloomery activity producing local iron. The main demand for the metal was for agricultural implements and other simple tools – NOT predominantly for weapons or warfare as we might think now. When we discover ‘archaeological’ metal artifacts such as pattern-welded swords or chain mail, we find that they have been produced by outstandingly skilled craftspeople and we are thrilled and amazed - BUT - iron nails do not fall into this exciting category. They are simple, crude and would have been produced every day by the local blacksmith. Rivets, roves, spikes were ‘local’, ‘ordinary’ and made from local materials, by local people.

 

If we could get a definitive analysis of the rivets used in the original ship It would be my guess that there would be a number of impurities due to them not being hammered out of the original low-grade iron being used. The many different iron type are named principally after the percentage of iron in the material used, Magnetite 70%, Hematite 65%, Limonite 35% etc. The Anglo-Saxons did not have chemical-breakdown tables to consult; they used what they had – used whatever was produced locally. We are not 6th century Anglo-Saxons. Maybe we are over-complicating things by thinking of chemical analysis? Our knowledge was not their knowledge.

We don’t know where the boat was built, but without evidence to the contrary we might assume that it was built somewhere within Raedwald’s sphere of control, an area that we now call East Anglia. It is possible that we might learn confirmation of this from the chemical composition of the impurities within the iron? Beyond this, the speculation would be that the iron used is just that – a softish iron, locally produced by Anglo-Saxon blacksmith shipwrights. I would not expect sophisticated additions, or that the metal was imported from very far afield. The ship was built for this man to travel around the area that he controlled

We know the bloomery method

We know the kind of iron it produced

We know the rivet-making process

I see no great technical mystery to be found in the rivets. Men with hammers could produce the metal then and, probably the same applies today. We just need low carbon, wrought iron.