Victorian and Edwardian Terracotta Buildings

ARTICLE

Victorian and Edwardian Terracotta Buildings

Blashfield's muffled kiln for terracotta and tiles
c. 1867

The last decades of the 19th century saw a proliferation of terracotta construction characterised by big, sumptuously ornamented, metal framed buildings. Many are now decaying. Jonathan Taylor examines the most common defects arising and the conservation approach

The term terracotta - literally meaning 'fired earth' - is generally used in architecture to describe a form of masonry made from moulded clay which is principally distinguished from brick by its relatively larger size and finer quality. When terracotta is glazed it is more correctly described as 'faience'.

A 'POTTED' HISTORY
By the Victorian period, terracotta already had a long and illustrious history as a form of architectural ornament, with examples ranging from ancient Greek temples and Renaissance churches in northern Italy, to Tudor stately homes. At the start of the period, the material was well known in Germany and Austria, and the occasional example could be found in this country.

By the 1860s a number of eminent English architects and intellectuals had recognised its value for mass-producing ornament and fine masonry by casting from an original, combining new technology with traditional craftsmanship. The material offered a new approach to style and decoration, founded on historic precedent, that suited the Victorians. The choice of the material for the construction of the Royal Albert Hall and the Victoria and Albert Museum in the early 1860s proved to be major landmarks, and in 1867 Sir Charles Barry presented a report to the Royal Institute of British Architects extolling its virtues. He showed that the material was light and easily transported; that it was strong in compression; that it was cheaper than stone particularly for the production of repeated decorative elements; and that its smooth, fired surface was more dirt resistant - an important consideration in the filthy urban environment of the period.

The earliest use of terracotta was as hollow pots filled with concrete or as solid blocks, and constructed in the same manner as brick or stone masonry. Whilst it continued to be used in this manner throughout the period, it was as a facing material for the new metal-framed buildings of the late 19th century that its development really blossomed. Following the fire which swept through Chicago after the earthquake in 1871, metal framed buildings were shown to be dangerous unless insulated and protected by masonry, as heat buckled the frames and the sudden cooling by fire-hoses caused the structures to shatter. Terracotta proved particularly suitable, with special profiles designed to accomodate metal sections at little or no extra cost.

The Wrigley Building, Chicago. Steel-framed and clad in graduated shades of white terracotta, this 30 storey skyscraper is a subtle advertisement for Wrigley's chewing gum, constructed in 1924, and recently repaired using terracotta made in the UK by Ibstock Hathernware. (Ibstock Hathernware Ltd.)

MANUFACTURE
Essentially, the process involved the production of clay 'models', constructed at a twelfth over-size to compensate for shrinkage, from which plaster moulds could be made. Fine fire-clay was mixed with up to 15 per cent 'grog', consisting of fired and finely ground clay to reduce shrinkage, and was carefully packed into the moulds. It was then left to dry, the clay shrinking slightly in the process so that it pulled away from the mould, allowing easy removal. Some manufacturers then finished the piece by hand, using a blade or damp cloth to smooth the moulded surfaces, while others preferred a more coarse, natural finish. The piece was then moved to paper lined shelves to dry further for up to a week, and any glaze required was applied at this stage, before firing.

The first kilns were 'muffled', conical, brick kilns (see illustration) which were gradually replaced by the 'down-draft' kiln in the mid to late 19th century. Although in the muffled kilns the wares were protected from the sulphurous flue gasses by a thin skin of brick, essentially the firing process was the same. Once the coal fires were lit, the heat was gradually increased over the first day to ensure that the clay dried out fully, without shattering. The kiln was then heated to full temperature at around 1,000º to 1,250ºC for eight to fourteen days, during which the silicate particles of the glazes fused or 'vitrified' like glass, with limited vitrification occurring in the clay bodies themselves. In unglazed material, a significantly higher degree of vitrification occurred in the finer particles of the surface, producing a thin, dense layer known as the fire-skin, which is crucial to the durability of the material. The kilns were then allowed to cool slowly. The process was a hit and miss affair, with considerable variations in temperature both within the kilns and between firing, particularly in the early years. Some pieces were consequently underfired and soft, leaving them vulnerable to decay, while others were over-fired and brittle due to excessive vitrification.

Faults evident in terracotta construction today often result from the firing process. However, not all faults are cause for concern. Minor cracks and variations in form and colour are often a product of their manufacture, which do not affect the performance of the material. 'Crazed' glazes, crisscrossed by fine hair lines, are particularly common but with no damaging effect. Underfiring, on the other hand, is more serious, and is the most common significant manufacturing fault. Others include iron pyrites staining, weak and poorly bonded glazes, and inadequate packing of the moulds which sometimes results in surface delamination.

CONSTRUCTION FAULTS
In the rapid urban expansion of the period, the Victorians experimented with new forms of construction and new materials which were often poorly understood and, at times, entirely inappropriate. In the terracotta buildings of the late 19th and early 20th century, stress cracks are common due to the use of a hard mortar of one part Portland cement to three parts sand, without movement joints, leaving the material vulnerable to differential movement between the frame and the cladding, settlement, vibration, and freeze/thaw moisture expansion.

Clinker, a light weight waste material produced by the blast furnace was also widely used as aggregate for the fill. It can expand when wet and was rich in sulphates and other salts. These dissolve and are carried through the structure. Wherever the moisture finds its way to the surface and evaporates, the salts accumulate producing crystals visible as a white surface bloom known as 'efflorescence'. Within the surface of the material the crystals expand causing surface failure.

Salts may also be inherent in the mortar and in the terracotta itself, and can also be introduced from other sources, including air-born pollution and even pigeon droppings. Chimneys are also particularly vulnerable due to the sulphate content of the flue gases. These salts may also react with Portland cement to form calcium sulpho-aluminate, resulting in physical expansion and hence further damage.

Stress may also be induced by the development of rust on ferrous metal fixings and framework, expanding in the process. Salts inherent in the fill may act as a catalyst, promoting corrosion. Balconies and projecting features such as cornices are the most vulnerable, and all terracotta buildings of the period which are constructed with a steel frame may be expected to display some sign of this form of deterioration.

Ironically, one of the most common forms of decay has been caused not by any inherent defects, but by our own intervention and the use of inappropriate cleaning methods in particular. The protective fire-skin is very thin and is more easily damaged than one might expect: once exposed, the softer, friable surface below commences an irreversible process of decay. Innumerable terracotta buildings have been disastrously harmed in this manner, including some of the most important examples in the country. Even the most carefully controlled cleaning methods may cause considerable damage when repeated several times over the years.

MAINTENANCE AND REPAIR
As with all buildings, the key to successful conservation lies in careful maintenance and minimum intervention. It is vital that all pointing, flashings, roof coverings and balcony drainage are regularly maintained, and rainwater goods should be cleared in order to reduce the risk of water penetration. New flashings may be needed to protect major projections, and bird control may be required. Where more direct intervention is unavoidable, each aspect of the work should be carefully considered and all possible consequences identified.

By comparison with other techniques of construction, our knowledge of the behaviour of terracotta and the construction techniques used remains quite limited. Yet it is clear that it is among the most vulnerable of materials. In addition to the vulnerability of its fire-skin, it is also relatively fragile, and it is often impossible to remove individual blocks fixed in Portland cement without smashing them.

Wherever possible, repairs should be carried out in situ to avoid the need to dismantle the blocks. But inevitably there will be areas in most projects where there is no alternative, such as where metal fixings and frames need to be treated or removed. Two companies still manufacture terracotta in this form in the UK, Ibstock Hathernware in Loughborough, and Shaws of Darwen, Lancashire. Both are able to supply new material to match original work. Alternative replacements such as reconstituted stone may be cheaper but provide temporary solutions only, for elements which can be replaced later, as they will not weather in the same manner as terracotta. Glass reinforced plastic (GRP) in particular is easily damaged and is liable to deteriorate in exposed conditions.

Recent attempts to consolidate the surface of the material where the fireskin has been damaged have met with limited success, and should also be regarded as temporary measures to prolong the life of material where replacement is inevitable. Both consolidants and water repellents may change the appearance of the material, and can actually increase the rate of deterioration by causing the surface of the terracotta to literally peel away through differential movement. If the surface is sealed, deterioration may also occur in surrounding areas, by promoting more concentrated salt crystallisation in these areas.

CLEANING
Although designed to resist the pollution of the industrial cities, terracotta soils badly, and in time, thick encrustations of sooty deposits build up, obscuring both subtle variations in shade and tone as well as rich polychromatic decoration. Cleaning not only achieves dramatic visual improvements but may also reveal cracks and other defects which might not otherwise be found. Hence cleaning may be both aesthetically desirable and structurally necessary.

Options include the use of non-ionic soap and water washing with fine mist sprays; low pressure air/water abrasion with micro-particle abrasives; and very dilute chemical solutions of either acid (hydrofluoric acid) or various alkalis. However, where terracotta is concerned, no method is completely safe, and many of our finest terracotta buildings display the scars of earlier cleaning attempts; alkalis may introduce salts; acid and even the most gentle abrasives inevitably etch the surface, damaging the fireskin; and water may dissolve salts already present causing further decay. In each case steps have to be taken to minimise risk of damage, and their success depends on the skill of the craftsman carrying out the work as much as on the suitability of the method to the particular conditions. Test panels may be of limited value where different craftsmen will be involved in carrying out much of the work.

Most recently, lasers have been developed and used successfully for cleaning terracotta, but at present are generally considered too slow and expensive to use outside a museum environment.

Despite its tough appearance, terracotta is one of the most vulnerable materials likely to be encountered in conservation work. There are a number of specialist companies with increasing practical experience, and for even the most minor repairs, thorough research and consultation is highly advisable.

FURTHER READING