165

Conservation by gam n ** the Masea of CentruZ Bohe

in Roxtoky

J. Urban

Born in 1918, graduated at the Prague Institute of Chemical Technology, 1946. Dr Techn., 1949, Ph.D., 1966. From 1956 to 1980 he worked at the Nuclear Research Institute, Ret, where his main field of interest was radiation chemistry and ap- plication of ionizing radiation to conservation.

P. Justa

Born in 1955, graduated at the Prague Institute of Chemical Technology, 1981, is at present working with the Museum of Central Bohemia. His main field of interest is rhe conservation of works of art by gamma irradiation and polymerization processes.

There is considerable awareness of the ecological situation in industrial coun- tries which threatens the existence of some of our fauna and flora, and yet the existence of pests which destroy historical objects in museum collections appears to go unnoticed. These pests are an increas- ing danger to objects made of wood, leather, textile, paper and other materi- als. In the collections of the Museum of Central Bohemia at Roztoky, approx- imately 50 to 60 per cent of the items are infested with wood-boring beetles and other pests. Unfortunately, we do not have adequate statistical data from other regions of our country or from other countries, but one can safely assume that the situation elsewhere is no better.

Drawbacks of conuentionaZ methods of treatment

Fighting pests involves both preventive protection of the endangered collections, and treatment by chemical methods. Under present conditions, both methods have their limitations and the best results can be achieved by combining them.

Preventive protection requires large storage rooms and depositories. As the number of collection pieces increases, the depositories (which are often unsuitable for storing museum collections) are becoming overcrowded and thus the danger of further infestation and damage of historical objects increases. Besides preventive protection from pests, active methods for their extermination are,, in- dispensable.

Up to now, chemicals were used for this purpose. The treatment with liquid insecticides is, from the technical point of view, easy to perform (by coating, dipp- ing or spraying) but the efficiency of this, treatment is problematic as 'the insec- ticide does not penetrate into the inner layers of the material. The killing of pests with a gas (e.g. hydrogen cyanide, hydro- gen phosphide or ethylene oxide) is more

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efficient, since these gases are applied at a higher pressure. However, this method requires pressure chambers and may cause problems with health authorities. Museums and conservation workshops are usually situated in populated areas. Health regulations require ventilating chimneys of at least 30 meter necessary for such fumigation as the rooms require. In most cases, this requirement cannot be met for reasons of aesthetic or town- planning. Nor does the fumigation of the depository as a whole give statisfactory results, as neither the necessary concen- tration nor the needed pressure for the fumigants can be attained.

Advantages of treatment by irradiatz'ovz

The disadvantage of all known chemical methods is their insufficient action in- depth. This drawback can be overcome by using ionizing radiation treatment, since it has a wide range of efficiency. Gamma radiation of the radioactive cobalt isotope 6OCo is penetrating enough to gq through even the bulkiest articles with a diameter of up'to 1 m and thus to exterminate pests in the whole of the in- fested object. A dose of 250 to 500 Gy is sufficient to rid the irradiated object of woodworm in all the stages of their life cycle. This leaves no harmful or health endangering residue in the object nor does it cause any damage to wood, polychrome, oil and tempera paints, sur- face coatings and glues, straw, textile, leather, paper and parchment. A dose of 500 Gy is small when compared with the low sensitivity of non-biological material to radiation and, if need be, it can be repeated up to twenty times without causing any damage to the irradiated object.

The important feature of the irradia- tion method (Fig. 41) is its productivity. In other words it is possible to gather together a large number of objects in the

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166 . J. Urban and P. Ju&

specialized irradiating chamber and to ir- radiate them all at the same time.

The relative drawback ofthe treatment is that it does not protect the object from further infestation. Irradiation should therefore be followed immediately by a preventive treatment of the surface of the irradiated object with a persistent insec- ticide. This surface coating should pre- vent the hatching of newly laid eggs and the infestation ofthe object with fungi or ~m~ulds ; it is not aimed at an in-depth penetration. It should dso be mentioned that objects may be irradiated when pack- ed in boxes of wood 01 metal, SO that the danger of damage during transport is reduced to a minimum.

The other possibility of using ionizing radiation as a means of inducing poly- merization in petrification procedures is not mentioned here, as this method can- not be used for a mass application. Petri- fication through radiation is now at the testing stage with regad to the commer- cid availability of the monomers needed and to the variability of the materid to be

petrified. This method requires different approaches in different cases and cannot yet be used in a routine way.

DeveloJmenat of¿%e g&amw8a ~ & a ~ ~ & a ~ ~ o ~ method

]From 1976 to 1980, a conservation ir- radiation facility (GW) was built for the Museum of Central Bohemia at Roztoky in co-operation with the Nuclear Re- search Institute at Rei. This facility is us- ed exclusively for the treatment ofcollec- tions of historicalgbjects. It is located in the basement of the former brewery of the castle and consists of five I Q Q ~ S with a total area of 120 m2. Bulky 01 heavy ob- jects are transported by four-wheeled trolleys. The dimensions of the radiation chamber are 4.5 x4.5 x 3.6 m (see Fig. 42). The original walls were 1 m thick and 20 an of concrete were added to increase the shielding power. The ceiling which is 125 cm thick was rebuilt from heavy reinforced conc~ete.

The other rooms are: the operator's

room, the store-room, the manipulation room, and the chemical conservation laboratory equipped with a large exhaust box used for the manipulation of the harmful substances. The shielding door of the irradiation chamber (Fig. 43) is made of cast iron 15 cm thick and is open- ed and shut electronically. The protec- tion afforded allows the original activity of the cobalt source (66 TBq) to be in- creased threefold. The GOCQ source is under the floor level of the irradiation chamber in an adapted transport con- tainer KIZ 10,000 made of lead and stainless steel. The source itself, together with a shielding plug, is pulled up into m irradiating position by wires and pulleys set in a~aotion in the operator's room.

The safety of the operation staff is en- sured by both electrical and mechanical blocking and signalling systems. This system does not allow the door to the ir- radiation chamber ro be opened when the source is in an irradiation position; nor can the source be moved into an ir- radiation position unless there is 180 one in the irradiation chamber and the shielding door has been closed. The ex- haust of the radiolytic products of air (ozone arad nitrogen oxides) Blows through tvvo ventilators which exchange the air in the irradiation chamber corn- pletely within either 33 or 11 minutes, ac- cording to whether the door is closed 01

open. This method for removing the ra- diolytic products has a safety factor of 10 so that one can enter the irradiation chamber immediately after the end ofthe irradiation with no health risk.

These safety measures are coupled with a conspicuous system of signalling lights which informs the operator about all situations in the irradiation chamber. It includes the lights in &e irradiation chamber and above the shielding door, as well as a number of lights on the control panel which indicate the position of the source, the functioning ofthe ventilators, and permission to close or to open the en- trance of the irradiation chamber. The movement of the cobalt source from the container and back can be checked on the television screen situated above the con- trol panel. The rest position ofthe source in the container is also indicated by the device measuring the activity background in the chamber (aradiometer), connected

41 Wooden objects prepared for irradiation treatment.

Conservatioiz by gamma radiation: the Museum of Cegtral Bohemia in Roztoky 167

with the control panel. The ‘one key’ safety system, excluding the possibility of incidental irradiation is moreover backed up by emergency -push buttons situated inside the irradiation chamber. These allow the source to be transferred im- mediately into the container and the shielding door to be opened.

Advantages of a conservation ~

irradiation faciZity (CW)

Considering that the CRF in the museum at Roztoky is probably the only facility of its kind located in a cultural institution, it would be useful to mention the advan- tage of the solution and the simplicity of its operation. A museum is not a very qualified milieu for this kind of activity and the conception of the technical design of the radiation facility therefore took this into account. First, the location in basement rooms made the construc- tion of bulky screening walls unnecessary and the historical character of the castle at Rostoky remained unchanged. Second,

BRICKS

42 Plan of the irradiation chamber.

the exchange and addition of new 6oCo sources, which is usually a difficult prob- lem, was solved very simply here. When a new 6OCo source has to be added, the container KI2 10,000 (which is con- structed for transportation) is uncoupled and transported to the Nuclear Research Institute at Reg, which is not far from Roztoky. Here the source is filled in a special pool under a 2.5 m deep layer of water. The whole operation including uncoupling, transport, filling and as- sembling takes three days.

The moving of the source into an ir- radiation position and back is carried out manually, with a mechanical device, in- dependent of the electrical power. This allows for the interruption of the radia- tion even if the supply of electrical energy is cut off. The returning of the source into the container is of the utmost importance for the operation’s safety. It is signalled in several ways and can also be checked visually on the monitor of a television camera. The hole in the container is pro- tected from blocking during irradiation (e.g. by a splinter of wood, a small ani- mal, etc.) by a cylinder made of hard alu- minium sheet which is moved by remote control from the operator’s room. In the case of a fire, the safe returning of the source to the container is also ensured; the sensors of the fire detector automati- cally start the electrical ‘emergency’ cir- cuit mentioned above, which automati- cally returns the source to the rest position in the container.

The conservation irradiation facility in

the Museum of Central Bohemia at Roz- toky has been working since 1982. Its capacity amounted to 1,850 objects of average dimensions per year.

The contribution of CRF from both the methodical and technical point of view may be briefly summarized as follows:

It is a new and progressive method of restoration introduced in an area where working procedures tended to stagnate. It was therefore necessary in the first place to overcome the conservative approach of our restorers, before they accepted the method and before they started to ap- preciate it as a highly efficient and at the same time harmless procedure which saves time.

This irradiation facility which was put into operation now meets all restoration requirements even in such ‘conservative’ surroundings as those of a museum. W

43 Entrance into the irradiation chamber showing the control devices.