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technical & practice

As the new series of the BBC'sRestoration begins on TV, Dr Jonathan Foyle, former assistant curator at Historic Royal Palaces, starts a new series of articles looking at the history and conservation of building materials. Part one: brick

Brick is probably the most misunderstood building material. Brick - the word sounds masculine in its solidity: 'You're a brick', 'Built like a brick s**thouse'. It is the permanent stuff of ancient Roman baths, of canals and warehouses, Victorian stations, vast chimneys. But it is also the fabric of Sussex farmhouses and the turreted silhouettes of Tudor mansions, and often requires careful conservation.

Brick in England started life as Roman 'tegulae', or wall tiles, of a shallow, long profile recognisable across Europe. Sufficient were produced to quarry for a thousand years, until native brick making recommenced in East Anglia, apparently during the 12th century (see Polstead Church, Suffolk). From the 1200s, durable yellow Flemish bricks, called 'klinkers', were imported, often for flooring.

(Their high quality has been subverted by the modern homonym of 'clinkers' - overfired, poor-quality bricks. ) The 15th century was the first great age of English brick production, when the material was promoted by bishops for their palaces, and Britain never looked back. Henry VIII's palaces absorbed bricks in batches of three million.

Early bricks are difficult to date by colour and size due to the localisation of manufacture, the variant shrinkages in turning out to dry, and their colouration and further shrinkage in early clamp kilns (like a flat pyramid of stacked air-dried bricks, interspersed with fuel, vented by integral air channels). The first legislation to standardise bricks came in 1571 in London, which had little apparent effect, and many rural houses were simply built from local brick-earth pits. In the later 17th century, as the Flemish 'cut and rubbed' tradition was matched technically in England, mortar courses often became thinner and bricks were more evenly manufactured and kiln-fired.

The 18th-century penchant for limestone led to the production of yellow stock bricks which characterise much of Georgian and Regency brickwork (for instance, Holkham Hall, Norfolk; Gloucester Terrace, London).

At the same time, 'mathematical tiles' disguised the archaic timber-framing of Sussex and Kent with the semblance of brick masonry. Technical innovations of the industrial age include the impressed 'frog' for greater stability in bonding, and Accrington bricks which are durable enough to be bound by hard cement. Italian immigrants produced rough and plentiful Flettons in factories near Peterborough.

It is its diversity and ubiquity which means that brick is not a single entity - its history has evolved, even on a local basis, to suit the economic circumstances and aesthetic tastes of those who paid for the buildings; the technology of those who dug the clay, shaped and fired the bricks; the skills of those whose hands laid and decorated them.

This universality means brick is often taken for granted as a practical material, especially so as the characteristic of brick which sets it apart from reinforced concrete or plastered timber-framing or stone is its repetitiveness as equal units from the bottom to the top of the building.

This character is, of course, represented in the bonds by which the bricks' laying pattern is manifested on the external facade. Brick bonds express the honesty of their loadbearing function far more than Corbusian reinforced concrete, and create an aesthetic so robust that the construction itself is the decoration.

Or so you might think? Lime colourwashes Perhaps the most important fact to remember with historic brickwork is that it was seldom meant to be seen, but for most of its history was covered with colourwash. Colourwash is basically limewash - that is, burned lime powder (calcium hydroxide) slaked with water and occasionally augmented by casein (milk protein), coloured with ochre, usually red ochre. The gloop is called 'ruddle', and was also used for marking sheep.

Ochre is a virtually ubiquitous earth mineral, and once extracted can be roasted from a deep yellow through terracotta to red and plum colours: think of the rich, warm colours of the streets of Rome.

In the Tudor period, we hear of 'pencilling' (from the Latin penicillus: brush) which required three stages:

first, a coat of ruddle; then fine soot from burned hay was mixed with lime and water into a black paste, and painted on the wall to resemble burned bricks in diamond patterns or 'diaper work'. The third stage was the replication of white mortar courses, using pure lime and water.

If human beauty is only skin deep, the same can't be said for brickwork as our concept of beauty in historic buildings changed permanently in the 19th century with an epidemic of wall scraping to reveal the bare bones of the noble structural materials. In this phase, the 'skin' became dispensable. But if a building was ever colourwashed, if you look closely beneath sills, cornices, and pediments, behind drainpipes, more often than not some fragments will have survived weather and redecorations.

Diaper work

The familiar diamond-patterns to be found in brick buildings from the 15th and 16th centuries are generically called 'diaper work'. The patterns are made using overfired bricks which feature an incidental ash glaze caused by the vaporisation of wood ash at about 1,200infinityC that fused to the faces of bricks nearest the burning fuel of a clamp. Though restorations have frequently replaced glazed bricks, the genuine articles are discernable by an aerosol-sprayed effect at the edge of the bottle-green glaze. Fakes are either brushed or dipped in thick, even glazes.

As laid, the patterns of glazed bricks seem to have little regularity, but feature different sizes of diamonds, sometimes motifs such as crosses, and are often stratified as if bricklayers put down their tools at the end of one season and started a completely different pattern the next. William Waynflete's entrance tower at Farnham Castle, Surrey (circa 1475) is unusually consistent. Many diaper-patterned walls were also colourwashed or even pencilled, which would suggest that diaper work was not meant to be seen. The obvious response is 'why bother?', and all suggestions would be most welcome.

Pointing to a certain age

Pointing can be a good indicator of the date of a brick building, but the chances of finding original pointing are slim. Roughly speaking, late medieval and Tudor work is characterised by 'double-struck' pointing, that is, a convex angled profile which casts a shadow on its underside. It emphasises each brick so that facades have a complex, almost pixellated, texture. By the mid 17th century, this technique had been replaced by 'penny-ruled' pointing in which the mortar was flush with the brick face, except for a groove of perhaps 2mm to 3mm in width and depth impressed along its centre. When colourwashed, this technique gave facades the appearance of tightly laid bricks which contributed to a smooth, monolithic effect.

Though the term 'cement' is medieval in origin, Portland cementbased mortars enter the scene from the early 19th century, and on historic buildings are often of the 'black ash' type, tinted with domestic fuel ash to match the sulphuric pollution of this industrialised age.

Case study: Kew Palace

Kew Palace presides over the northern lawns of Kew Gardens, close to the Thames. It was built about 1630 by a merchant and his wife - Samuel and Catherine Fortrey - and became a palace only after it was leased to Queen Caroline in 1728.Under Historic Royal Palaces, its brickwork underwent conservation in 1996-98.

The basic philosophy was to retain as much of the original fabric as possible.Contriving the impression that nothing has changed can be expensive: the external repairs have cost £1.2 million.

The designer and builders of Kew Palace are not recorded but it is nevertheless an important building. It is the earliest known native example of a consistent 'Flemish bond'composed of alternate headers and stretchers. (The name is confusing: Flemings usually built in English bond, of alternating courses of all stretchers and all headers, but the Italians sometimes used Flemish bond; for instance, Palazzo Venezia, Rome, c1456).

It is often called the 'Dutch House' for its ogival brick gables but again confusingly: Dutch gables are normally stepped and are universally capped in stone.Kew's facades have classical overtones, with pilasters and rusticated and pedimented architraves, and the second storey features columns uniquely made of brick laid on end with no reinforcement.

The first task was to make an assessment of the building's phasing.The job was made easier because every generation of repairs had a significantly different character, and seven main types of bricks were discernible, spanning three centuries of work.Each sought to replicate what had gone before, and so much of the original material had remained in situ.The original lime mortar was easily identifiable (less so where the gables had been rebuilt at an early date).

With an understanding of the building's phases of construction and repair, and of the potentially weak junctions between them, an assessment could be made of its structural problems.Cracks either side of the south-east corner showed it was separating from the adjacent walls so that the angle had become a pier.

This was explained by the fact that the adjacent walls had been rebuilt in the 1840s while leaving the original corner intact, presumably to hold the floor decks in place.The rebuilt walls had since settled.The two elements were stitched together by a series of stainless steel dowels within nylon 'socks', which were inserted into cavities drilled into mortar courses spanning the cracks and then syringe-filled with lime grout to fit the cavity precisely.The cracks were similarly filled, whereupon the whole then acted as a single structural mass.

The north wall had been relatively sheltered from the weather and retained the original pointing and colouring.The removal of a lead downpipe bracket revealed an area of original penny-ruled pointing. Its grooves still appeared fresh, and what's more, they still featured the primary colourwash of orangey ruddle.Above the south door, once protected by a 19th-century porch, was a later thick layer of dark red ruddle that had presumably been tinted to match the polluted wall. In all, four generations of colourwash were present.

Several of the gables were in a sorry state, a typical result of their exposed situation which makes it unusual that these survived.

Contemporary hand-made bricks were produced by Lamb's in Sussex to match the originals' typical size and depth of firing.

Where necessary, the original brickwork was dismantled and bricks labelled for reincorporation, while trial gable pediments were built in the works yard. Much skill was involved in contriving the right level of wonkiness consistent with the survivors'360 years of ageing.

Repointing was a cause of debate. Cement mortar damages bricks by its imperviousness to water, while lime mortar and bricks are vapour permeable. When moisture collects between bricks and cement pointing, it creates stress, particularly on south-facing elevations vulnerable to freeze-thaw action.These molecular forces can delaminate the faces of bricks (try tapping with your knuckles on south and north facades of cementpointed old brickwork). In raking out the 19th-century black-ash pointing, which could be removed without damaging the bricks it adhered to, the original soft white lime mortar bedding with characteristic chalk lumps was revealed.The conservation work would use lime mortar because of its homogeneous qualities, but a decision had to be made on whether to match the colour of the original mortar or tint it to match the black ash cement that unavoidably remained.Samples were made of lime mortars using various sands, and Chertsey sharp was chosen at a ratio of one part lime to two parts sand. It was decided to leave the mortar creamy white.

Upon completion of the repointing, the predictably piebald effect was akin to a moth-eaten lace curtain draped over the building. It had already been mooted by the project team that the discovery that ruddle had been used for about two-thirds of the building's life might justify its reinstatement, which was supported by English Heritage.

Trials then began by Richard Roberts, conservation surveyor, on the ratio of lime to water, on the usefulness of adding milk for its casein and the colouration of the ochre pigments supplied by St Blaise. After regular hosing to simulate weathering, it was clear that using two coats of casein-bound ruddle matching the earlier layers was the best solution.As the first layer dried, a white bloom and uneven colouration spread across the facades.There was no going back but fortunately the bloom was brushed off and the unevenness settled. Armed with the solecism that any problems were likely to be historically authentic, the work was completed in January 1998. Love or hate the result (but seldom between), it must be admitted that the new vogue for old ruddling has yet to catch on.

BRICKWORK CONSERVATION 1996-98

CLIENT Historic Royal Palaces PROJECT MANAGERS Philip Hartley and David Farrington CURATOR Jonathan Foyle CONSERVATION SURVEYOR Richard Roberts ARCHITECT Donald Insall Associates: Tony Dyson and Francis Maude CONTRACTOR Higgs and Hill: Mike Warrington STRUCTURAL ENGINEER Alan Baxter: Stuart Tappin RUDDLE SUPPLIERS Rose of Jericho RUDDLERS Chichester Cathedral Works

Further reading:

James W P Campbell and Andrew Saint, 'A Bibliography of Works on Brick published in England before 1750', Construction History Vol. 17, 2001, pp17-30.

Nathaniel Lloyd's 1925 A History of English Brickwork. This classic has just been republished (2003) by The Antique Collectors' Club and retails for just £35. Subtitled 'with examples and notes of the architectural use and manipulation of brick from mediaeval times to the end of the Georgian period', this is a masterly work, written with intelligence and rigour. It covers practically everything and it is replicated in its original interwar font and layout. This can lead to it being thought of as increasingly archaic, and to be replaced by the (unfortunately unreferenced) Brick Building in England by J A Wight (1972) and Ronald Brunskill's more recent and straightforward Brick Building in Britain (1990).

Many scholarly articles exist on early brick by T P Smith, eg 'Rye House, Hertfordshire and Aspects of Early Brickwork in England', Archaeological Journal Vol. 132, (1975) with useful bibliography.

James Campbell and Will Price have recently produced the fine Brick: A World History in Architecture (2003) while Gerard Lynch has produced practical and technical guides to building in Brickwork: History, Technology and Practice or Gauged Brickwork: A Technical Handbook.

For a major international exploration, read N S Baer, S Fitz and R A Livingston's The Conservation of Historic Brick Structures.

Serious enthusiasts may like to join the Brick Section of the British Archaeological Association (www. britishbricksoc. freeonline. co. uk).

Enquires on brick, its history, conservation and supply are also welcomed at the Brick Development Association (www. brick. org. uk).

The current edition (Volume 25, Number 2) of Cornerstone, the renamed magazine of the Society for the Protection of Ancient Buildings (formerly SPAB News) carries a special edition on brickwork (www. spab. org. uk).

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