Last edited 21 Nov 2021

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Institute of Historic Building Conservation Institute / association Website

Conserving terracotta

For all the 19th-century claims about its durability and self-cleaning nature, terracotta is little understood as a material and experience in its conservation is limited.

I blame it all on the loveable Dennis Moffat (d2010), bless him.

My 36-year love affair with terracotta and faience started with a plaintive call from the front desk at County Hall, where I was hiding out in the attic offices of the GLC’s Historic Buildings Division. In a desperate last move for a salesman out of ideas, and luck, Moffat was traipsing the capital’s streets looking for Edwardian piles with glazed brick courtyards: did we want to buy any replacements? If not, then one of the last great factories producing architectural ceramics would close. The rest, as they say, is history.

Since just before and after this event, circa 1980, the two main English manufacturers of terracotta have seen their prospects plummet then soar on more than one occasion. Such is the tenuous grip on commercial success for dedicated craftspeople in an uncaring fiscal environment. Receivers, accountants, banks and commodity brokers have stripped them bare of diversified product ranges (which used to protect them from economic downturns in the construction sector). Famines and feasts follow hard on each other’s heels: with a currently buoyant refurbishment market, continuing overseas demand, and resurrected new-build interests all coming at once, the small-and-medium-sized businesses are now stretched almost to capacity. All power to their elbows.

Before recent contributions to Historic England’s epic two-volume blockbuster on the subject [1], I last wrote an overview of terracotta conservation issues, ‘Fragile remains’ [2], some 22 years ago. Sadly, the basic tenet of that paper remains valid today: terracotta and its associated construction systems remain the least understood of any historic building material. On this score, wheels are constantly being reinvented – witness my former English Heritage colleague, historian Anna Keay (now a TV star and director of the Landmark Trust), recently poring over a boffin’s microscope on the box to discover the long lost secret recipe of Mrs Coade’s ‘stone’. This ‘myth’ of unknown constituents I put down to blissful ignorance or clever marketing by Alison Kelly for her 1990 book [3]: the GLC and others carried out scientific analysis of the material back in the 1950s, and potters throughout history have known about and used grog (previously fired clay) to stabilise their products.

Knowledge gaps are so widespread that it is hardly surprising that damage has been caused by well-meant but inappropriate cleaning and other remedial treatments. Here, then, are a few salient issues to keep in mind.

General contractors sometimes try to financially squeeze the terracotta-fixing subcontractors and material makers by claiming that new blocks do not match the old because they are warped. But on most jobs that I have seen, the historic material (remember, shrunk down from 5–13 per cent of its original moulded size) has a very wide range of tolerances for squareness, flatness and so on because of problematic drying conditions or firing variations in downdraft kilns. These deformities need to be measured and the tolerances specified to ensure a match within a defined range.

Few self-coloured clays are now available, so the manufacturers invariably produce a neutral off-white/ grey body from selected West Country clays and spray it with a slip-stain (engobe) finish that vitrifies in the kiln to become an integral coloured fireskin. The colour, therefore, is only skin deep. A common misconception among specifiers is that terracotta colours are constant for any particular job. This was not the case, nor could it ever be. Colour variations always occurred, within certain limits, depending on the type of kiln used, firing times and temperatures, weather conditions, and the position and arrangement of clay within the kiln. So it is vitally important for conservation officers to agree with the designer-specifiers and manufacturers a range of colours that may be required for any restoration project involving replica units. Obviously the more colour variations there are, the more expensive the project becomes. It is pointless conditioning consents, or specifying new materials ‘to match’ without defining precisely what that means. For example, glazes (possessing colour, texture and gloss) may have been abraded by sand-blasting, or etched by acid cleaning, in the distant past – so what is the manufacturer to copy? Be specific.

New slip-stain finishes are securely bonded to the terracotta’s underbody during firing. Made of a watered-down and sometimes dyed version of the same clay, the materials usually have a good ‘glaze fit’ (that is, matching firing shrinkage, thermal and moisture movement properties). However, like all glazes fired in pottery (and like film-forming paints on woodwork), the slip-stains tend to thin out and bleed away from sharp edges in the kiln, and conversely congregate in thicker zones at internal 90-degree junctions – and these tendencies are mostly outside the ceramic engineers’ absolute control. The former issue leads to thin colouring on sharp edges, and the latter problem can lead to micro-cracking in thick glazes.

Glazes, of course, come in a whole host of different colours, opacities and textures: some purposefully created to provide a Craquelure (crazy paving) appearance. Most survive intact in the UK, thanks to the mild climate. Elsewhere, poor ‘glaze fit’ exhibited by crazing can be punished with spalling surfaces, and worse – algal growth under the microscopically lifted glaze, drawn into the surface by rain through capillary absorption.

Inevitably in this fast-moving world of just-in-time production, short lead-time contracting, and so on, with handmade materials in great demand (both for quantities and speed of delivery), foreshortened drying and firing periods can lead to quality-control issues such as dunting – unwanted shrinkage cracks (called dunts) in the clay bodies, caused by unduly rapid heating or cooling during the firing process. On a minor scale, in terms of extent and degree, these blemishes will be nothing to worry about – an ASTM test states that if you can not see them from 1.5 metres (5 feet), they practically do not exist. But in some parts of North America, where temperatures can reach down to -32°C (-26°F) and fluctuate wildly at the front and back end of winter, tapered dunts can potentially be jacked apart by frost action if water gets inside the blocks.

An irony shared with my former colleagues at Historic England is that British practice has shifted considerably over the years to more drastic repairs employing replacement terracotta features, rather than the use of mortar patch repairs and other less invasive treatments. That is not a bad strategy as such, and it is often aligned with parallel standards in commercial stone repair, but nevertheless it lacks some of the finesse seen at the conservator end of the market, does it not? Perhaps this is because available proprietary repair mortars and most custom-made, lime-based treatments have never strictly matched historic pot surfaces as well or for so long, if ever. But this contrasts with North America where, because of the daunting economic scale of some of the giant skyscraper rehabs, cheaper option requirements have quietly driven some sophisticated improvements in patching technology by firms like Jahn Mortars and Edison Coatings.

Hand-in-hand with the patching mortars goes the extensive use of Helifix DryFix anchors and smaller CINTEC anchors, both British inventions making great inroads in the USA and Canada.

The deterioration of the anchoring systems through steelwork corrosion is perhaps the most difficult building fault to locate and/or diagnose comprehensively without destructive analysis. Because of the risks of catastrophic failure and resultant falling hazards from the underside of high cornices and parapets, debates about the best methods of construction and repair have gone on for more than 100 years on both sides of the Atlantic. Of course, everyone has migrated to replacement stainless steel fixings: fair enough.

But questions remain about whether to lock the steel in place by traditional means (such as dry-pack mortars and grouts), or to employ the twenty-first century rain-screen cladding technology of dry-fix frames, epoxy bolts and nylon washer pads. The jury is still out on this one, but I suspect that the credibility of traditional methods may be in long-term jeopardy due to some massive law-suit failures in the USA recently. These have been mostly the result of inappropriate deployments of cementitious grouts without adequate precautions in appalling weather conditions. Ignorance is not bliss.

To summarise: for all the nineteenth-century claims about the durability and self-cleaning nature of terracotta, the material is no more invincible in the twenty-first century urban environment than many of our widely used building stones. The forms of construction used traditionally with terracotta are as intricate and varied as any found in history. It is both a material and building system that is little understood and experience in its conservation is more limited than its use and distribution in our cities warrants or deserves. Although interest in the material is increasing, very few terracotta buildings have been adequately conserved.

Beneath the first few millimetres of tough glaze or fireskin, the underbody material can be quite soft and susceptible to erosion. Durability tends to be a function of clay mineral type and resultant firing temperature: for example, pink/buff coloured terracotta made from kaolinitic clays have to be fired at low temperatures and are the least durable; red and dark brown terracotta are fired at higher temperatures and are the most durable. So preservation of the surface of terracotta can be essential to its wellbeing. This has implications for its cleaning, consolidation, repair and maintenance. Our Victorian cities are adorned by many majestic ornate terracotta edifices, the like of which we are only now learning to build again [4].

References


This article originally appeared in Context 143, published by the Institute of Historic Building Conservation (IHBC) in March 2016. It was written by John Fidler, a former conservation director of English Heritage, who runs an international consultancy firm focused on preservation technology in Los Angeles. He was the primary author of BS 6270: 1986, Part 1, The Cleaning and Surface Repair of Buildings, Amendment G, ‘Terracotta and Faience’, and he chaired the development of its successor standard, BS 8221/2: 2000.

--Institute of Historic Building Conservation

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