Jim Spriggs is the conservator for the York Archaeological Trust and Yorkshire Museum. In the next few issues he will be describing the type of work carried out in his laboratory, and explaining the principles behind some of the treatments and methods used in the conservation process.

At a safe distance from the nerve-centre of the Trust in Aldwark, in fact at the other side of town, there is a small laboratory. This is the Trust’s and the Yorkshire Museum's conservation unit, situated in the basement of the original bathhouse of the now ruined St. Mary's Abbey. As far as we know, it is the only medieval laboratory in the country, but with this vision in mind, the visitor to the lab will have a pleasant surprise. On descending into the depths of the gatehouse, he will find himself in a bright and cheerful series of rooms, filled with the hum and click of scientific equipment and the faintly clinical smell of organic solvents. Among the microscopes and chemical reagent bottles, there are rows of carefully labelled plastic bags stroked in trays and lined up on shelves, each bag containing a broken big corroded relic of the past. These seemingly worthless scraps are in fact one of the major sources the archaeologist has for dating and ascribing some sort of function to his site, and also for providing an insight into the everyday life of the local inhabitants. These bits and pieces form a large percentage of the ‘finds’ dug up on the excavations, and we take as much care to examine and preserve these apparently insignificant remains as we do for the occasional more exciting and complete objects.

The visitor, at first slightly overwhelmed by the array of strange equipment and interesting-looking items in various stages of treatment, may pick up some recognisable object and enquire about its history add how it is being treated. The straight description of a conservation treatment would only be half the story, since the type of treatment to be applied depends largely on the way the object has corroded or decayed. So to answer the question fully, we should first explain how and why the object came to be in its present state of deterioration.

Most of the inorganic materials used by man have been won from the ground or otherwise transferred from their natural state to be made into artefacts. When the objects become lost or abandoned and return to the ground, they will slowly tend to revert back to their former natural state. Typical examples of the process are the metals, which tend to rust and corrode in the soil, forming oxides, chlorides, carbonates and sulphates similar to the ores from which they were originally derived. Similarly, glass, which is composed of sand and soda or potash will tend to disintegrate into these component parts due to the action of water and mild acids in the soil. This reversion to natural compounds may continue unhindered until there is no object left, save a mass of corrosion or a stain in the soil. Very often however, the process will became stifled or slowed down to such an extent that the object will reach a state of equilibrium with the soil environment. How much deterioration will have occurred before a state of equilibrium is reached depends on the prevailing soil conditions. As if this was not bad enough, when the object is dug up, the state of equilibrium will be upset again by exposure to a new set of conditions, and the process of deterioration will resume until complete disintegration takes place of a new state of equilibrium is reached. This second period of deterioration may be even more damaging then the first, especially as the object will already be in a fragile condition, and for this reason we are fortunate in having our own laboratory to which particularly susceptible finds may be taken almost the moment they are discovered.

Although comparatively small, and staffed by only two people, the laboratory is capable of dealing with several hundred finds each year, that is, most of the material produced form the Trust’s excavations. A proportion of time is spent on objects from the collections of the Yorkshire Museum and also for other excavations not fortunate enough to possess conservation facilities. There are, of course, occasional objects which, because of their size and complexity or due to our lack of time or specialised equipment, cannot be coped with. These are farmed out to other laboratories better equipped to deal with them.

During the course of the excavations, batches of finds arrive regularly from the sites and are immediately sorted into groups, some requiring attention more urgently than others, depending on their condition and stability. Those that are to be stored for a while are checked to see that they are properly packed. For example, waterlogged wood and leather must be stored wet or they will shrink and crack, whereas metals should be stored dry to prevent further corrosion. Since we have so many finds to deal with, we tend to save up groups of objects of one material so that they can, as far as possible, be treated together. That naturally will save time and chemicals, though conveyor-belt conservation cannot be taken too far as every find is worthy of individual attention. The actual process of conservation may be roughly divided into the following parts: examination; cleaning; stabilisation; consolidation and reconstruction.

Apart from the intrinsic value and stylistic interest of an artefact, there is much associated evidence which may be of equal value and interest. Microscopic examination of, say, a bronze brooch may reveal textile remains around the pin which, being preserved from bacterial decay by toxic copper corrosion compounds, will give information about the clothing being worn. Similarly, examination of the pattern on the front of the brooch, as an the case of the Anglian cross brooch from the Cattle Market excavation, (INTERIM Issue 1-4) revealed traces of the original red and yellow enamel decoration which over-hasty cleaning would have completely destroyed. Corrosion products themselves are of interest as they give clues to the original conditions of burial. For instance, the bluey-grey patina, called vivianite, which occasionally forms on ironwork, indicates a high phosphate content in the soil possibly due to the close proximity of skeletal material which may no longer be recognisable. Another type of examination, which ironwork particularly will be subjected to, is radiography. Since iron corrodes in a somewhat haphazard manner. X-ray photographs are often necessary to indicate the true shape of an object. These photographs will also show up any surface decoration - either incised or inlaid - and will even indicate how much actual metal is left in the core of a badly corroded object. Other types of examination which involve the chemical and physical analysis of the fabric of the artifact will normally happen after cleaning.

The next step, after cleaning, is probably the most crucial in the whole process for most types of material. Out of the many available methods, both chemical and mechanical, for cleaning metals, the choice of the wrong method could cause the loss of all surface detail and even the total disintegration of a fragile, badly corroded object. Chemical and electrolytic methods are comparatively fast and are designed to remove all dirt and corrosion down to the bare metal. Many metallic objects, however, would be sadly diminished in size and shape by these methods because of the small amount of remaining metal - hence the value of the X-ray photographs for making the decision. Formerly one has to resort to the time consuming mechanical methods, that is, cleaning by hand. This involves the painstaking picking, scraping and grinding away of corrosion, a little bit at a time, usually under a low-power microscope. Fortunately modern technology has produced two tools which greatly facilitate the process. These are the vibrotool, and the air-brasive unit. The vibrotool was originally designed for engraving and consists of a hand-held vibrator fitted with a sharp needle. Although it makes a hideous noise, which visitors to the laboratory may have had to endure, in the right hands it is an extremely delicate and sensitive instrument for chipping away corrosion and concretion. Rather more sophisticated and far more expensive, the air-brasive unit removes corrosion by bombarding the surface with abrasive powder at high pressure. This as an extremely delicate and versatile tool which may be used on the hardest concretion down to cleaning metal threads in a fragile textile.

In the next issue, Jim Spriggs will continue to explain the various steps in the conservation process, and describe some of the other activities of his department.

Jim Spriggs