EXCAVATION & CONSERVATION
|For 221 years the wreck of the Invincible lay undisturbed. Her contents, covered and preserved by the seabed sand, remain in much the position, as they were when the ship rolled onto her side on 19th February 1758. The Invincible was fully stored. She was expected to be away from port for several weeks and away from home base for several months, so every inch of space would have been filled with spares and provisions of one kind or another.|
|Left - the remains of a leather shoe with the original stitching holes still clearly visible. The heel is still attached to the sole by small 'oak' nails.|
|The purpose of excavating the wreck is to recover the contents of the ship to build up a picture of life on board. Layer by layer, compartment by compartment, the ship was emptied. By carefully recording and interpreting this work it is possible to identify what each part of the ship was used for, who worked or lived in her and what were the familiar every day objects with which they shared their lives.|
An archaeological excavation is a painstaking and difficult task at the best of times. When the site is 3 miles from the shore and in 20 feet of water it is even more so. In bad weather, divers and crews are often wet, uncomfortable and sometimes seasick. The water in the Solent is usually murky and cold - 20ft visibility is a luxury, 10ft is good, 12 inches all too common as you see on the right.
The method of excavation is to carefully disturb the seabed with the hand or a trowel and then to suck the disturbed material away with an 'air-lift'. (See image below) Gradually the artefacts become exposed. As they do so they are drawn, photographed and when completely free they are lifted. From the seabed they are taken to the dive boat and then to the shore base to be cleaned and conserved.
|Objects lying on the seabed are attacked in three ways, physically, chemically and biologically. The action of the tide causes the objects to be buffeted about and some become broken up by collision with other seabed debris. Some materials are, to various degrees, soluble in seawater or, as is the case of metals, corroded by it. Organic materials may be eaten by wood boring creatures such as gribble worm or attacked by marine fungi's and bacteria. Wood and leather artefacts, in particular, do not survive long on the seabed.|
|It is a source of constant amazement, to professionals working in the field and to interested onlookers alike, that so many artefacts have survived for so long and in such good condition. The explanation lies in the burial conditions, for only inches below the surface of the seabed the environment undergoes a massive and fundamental change. On the surface the colour of the sand is white, but as soon as the top few inches are removed the colour becomes black. To a chemist this colour change indicates that when buried, the objects lie in a completely anaerobic environment. In other words there is no oxygen present.|
|The blanket of sand excludes the action of the tide. Chemical action, mainly corrosion of metals, is slowed down almost to a stop by the absence of oxygen and the gribble worm and bacteria cannot live without it. Thus, in those parts where the burial conditions are stable and the objects are permanently covered, the site provides almost a perfect preserving medium. In this state of equilibrium the ravages of time are suspended and the wreck and its contents pass through the ages unaltered.|
|The arrival of archaeologists disrupts everything. The protective blanket of sand is drawn back, exposing the artefacts to the forces of nature again, but this time there is a new enemy; man! The degree of degradation is rarely consistent. Some objects will be in a good and robust condition while others will be fragile. The process of excavation, no matter how carefully done, represents the most vigorous action the objects have undergone in hundreds of years and damage in inevitable.|
|As the objects become uncovered therefore, the need for conservation arises immediately. They must be handled carefully and protected from the effects of the massive change in environment, that is, from a wet anaerobic one to a dry oxygen rich atmosphere. After recovery the objects are put into pre-treatment storage which will keep them in good condition until they can be stabilised. This stage of pre-treatment is potentially the most dangerous. Processes of corrosion and decay can be set in motion, which are extremely difficult to reverse. Storage arrangements can never be wholly satisfactory, but represent a set of compromises full of pitfalls. The period for pre-treatment storage therefore, should be as short as possible.|
|The stabilisation process will change the physical or chemical makeup of an object from one, which is stable in its burial environment to one, which is stable in normal living conditions. Which process is used will depend on the type of material under treatment and the degree of degradation.|
|After the stabilisation process is complete the objects can be kept in a normal living environment. However, extremes of temperature should be avoided. Organic materials may shrink and crack in a very dry atmosphere and support fungal growth in a very humid one, which will also promote corrosion in metals. The ideal situation is an atmosphere, which remains fairly continuously in the mid-range, this will be comfortable for both people and objects alike.|
|To assist in the learning process destructive analytical techniques can
be used, not on whole artefacts but on the many hundreds of fragments of
artefacts that were found on the wreck. Scientists at Portsmouth Polytechnic
have been able to identify: -
From divers to scientists, many and varied skills were combined to recover, preserve and interpret the past as it survives on an archaeological site. By doing so, future generations will benefit from the knowledge gained providing a greater understanding of past cultures and traditions which have helped to form our own.