UKIC Stone and Wall Paintings Section

Recent examples of material from the UKIC Stone and Wall Paintings Section pages of Conservation News published by the UKIC.

If you would like to recieve Conservation News , the quarterly magazine produced by the United Kingdom Institute for Conservation, along with the bulletin sheet Grapevine , and the annual journal The Conservator , click here to enquire about membership and publications.

Contents [Last updated 23rd January 2003]

Abstracts of research submitted on the Courtauld Institute of Art MA in the Conservation of Wall Paintings, Summer 2000.

1. Application of a protective plaster layer for in-situ preservation of exposed archaeological wall paintings at the Hippodrome, Caesarea, Israel

Karolina Tibensky

2. Artificial copper chloride pigments: methods of production and analysis

Sophie Godfraind

Application of a protective plaster layer for in-situ preservation of exposed archaeological wall paintings at the Hippodrome, Caesarea, Israel

Karolina Tibensky

Undertaken in collaboration with the Israel Antiquities Authority and sponsored by English Heritage

Exposed archaeological wall paintings are subject to deterioration due both to environmental factors and human activity. The highly important podium wall paintings in the Hippodrome at Caesarea, excavated from 1993 forward, are presently actively deteriorating. The main causes of decay include salts, moisture, biodeterioration, abrasion by wind-blown sand, and mechanical damage from visitors.

Standard solutions to such problems—such as sheltering, reburial, or remedial treatments—were rejected by the panel of international experts consulted in the matter. Sheltering would compromise the structure aesthetically, would not be feasible for the extensive and fragmentary remains, and, moreover, would not protect the fresco from many of the deterioration processes. Likewise, reburial would be unacceptable aesthetically, would not be feasible for the widely dispersed fragments of paintings, and, in addition, remains problematic for vertical surfaces. Surface treatments provide only short-lived solutions, since they do not address the causes of deterioration but rather their effects.

The present study explores an alternative conservation intervention: preservation by the application of a protective render. Specifically, a system of two plaster layers was devised. To expose the trial renders to the site-specific deterioration agents, plaster tests were applied on the original fresco and subjected to accelerated deterioration as well as the natural seasonal climatic changes.

Locally available materials were employed for the test renders (eight renders types for layer one and eleven for layer two); lime was used as the binder, and arad sand, crushed stone and ‘desiccant’ were the main aggregates. In addition, for the purposes of marking, pozzolana, ceramic, and coloured fibres were added in small proportions to the layer one renders. To reduce the density and increase porosity, light-weight fillers (Promaxon T® and perlite) were added in small quantities to some of the trial plasters for both layers.

Following a 10-month exposure period during 1999-2000, in-situ assessment was carried out including graphic and photographic condition recording, uncovering tests, and sampling for off-site assessment and testing.

Subsequently, the following procedures were undertaken in the laboratory to investigate the performance characteristics of the trial renders, and thus their suitability as a covering material. Particle size and morphology of the raw materials were characterised by sieving and microscopic examination of thin sections. Aqueous extractions of ions were prepared and analysed using ICP-AES and IC for both the raw materials and test renders. Density, effective porosity and water vapour transmission were measured in accordance with standards. Information about mechanical strength of the renders was provided by tests of uniaxial compressive strength, measured in a compressive test machine.

The research showed that test renders with a small particle size and a lime : aggregate ratio of 1:2 were easier to apply and had better adhesive properties. For layer one, renders with a larger aggregate size—that is, those containing crushed stone or desiccant—were easier to remove. A trial chalk intervention layer did not facilitate, but rather hindered removal. For layer two, renders containing sand proved to be aesthetically desirable. The markers used within layer one were not sufficiently visible as they were used in too small a quantity. However, the colour difference between the layer one and layer two renders proved to be sufficient for monitoring purposes. The porosity and effective density of all renders was comparable. Thin-section analysis showed a possible failure of the lime matrix in renders with a large aggregate size, making them possibly less desirable than the sand-based renders. Water vapour transmission proved to be higher for layer two renders, especially those with large aggregate sizes and greater aggregate ratio. Uniaxial compressive strength tests showed all renders to be weak, with renders for layer one slightly stronger than those used in layer two. Biodeterioration along the edges of some plaster tests indicates that during application good adhesion of the protective render must be achieved.

September 2000

Artificial copper chloride pigments: methods of production and analysis

Sophie Godfraind

Sponsored by English Heritage

Although copper chlorides are rare in nature, they are frequent by-products of copper corrosion. Their brilliant colouring found favour with artists and they were used for painting up until the mid 19th century when they were finally replaced by more stable pigments.

Both chronologically and geographically, copper chloride has a wide distribution: it is found in Egyptian painting of the Old Kingdom, throughout medieval Europe, and in Asia. The earliest known recipe is Chinese, and dates from the 8th century. From medieval Europe various recipes exist, all variants on a single basic method of synthesising the fines t and most stable product.

Identification of copper chloride as a pigment in paintings is hampered by the fact that it can also occur as an alteration product of other copper-based pigments such as azurite. The conditions under which such alteration occurs are not conclusively established, but it is most common, for example, in maritime environments and (apparently) in the presence of liquid water and chloride ions (as in some cleaning operations). Differentiation between primary copper chloride pigments and secondary alteration products is difficult, though combined analytical techniques together with in-situ evidence may assist in their distinction.

The research described in this dissertation explored some of the possible approaches to differentiating primary and secondary copper chlorides. Investigation of literary sources and a study of various copper chlorides samples—including natural minerals, samples produced from a range of recipes, as well as samples from historic wall paintings—suggested some profitable approaches to establishing appropriate methodologies. Morphology and optical properties (determined with polarising light microscopy) were found to be a satisfactory means characterisation. Although it had been suggested that the compounds associated with ‘bronze disease’ might indicate a secondary product, these compounds were also identified in primary pigments, so their presence cannot be considered conclusive evidence of alteration.

Further research, using a broader range of historic samples and controlled synthetic products, may aid in the resolution of problems associated with the identification of copper chloride. Understanding the range and use of copper chloride pigments, and the factors affecting their alteration, would not only add to our knowledge of the techniques of painting, but would prove invaluable in assuring their preservation.

July 2000