Architectural Terra Cotta

Design Concepts, Techniques and Applications

Donald B. Corner, John Rowell, Donald Corner

• Publisher: Routledge
• Year: 2022
• Print ISBN: 9780367178260
• eBook ISBN: 9780429557064
• Topic: Architecture
• Subtopic: Architecture General

***Winner of the 2023 BTES Book Award***

Architectural Terra Cotta examines the evolution of terra cotta and prepares architects and builders to make new, creative uses of the timeless material. Terra cotta is among the oldest of manufactured building products, yet it has once again become a material of choice in contemporary façade design. From the walls of Babylon to high performance rainscreens, terra cotta claddings have repeatedly proven to be technically superior and aesthetically triumphant. Understanding the evolution of terra cotta prepares architects to add new, creative chapters to a rich history.

This book describes the key attributes that recommend the use of terra cotta and explain its continuing success. The core of the book traces the many ways that terra cotta can be formed, finished and applied to buildings. These techniques demonstrate the full potential of the material, showing how its unique capabilities have been developed over time. A comprehensive inventory of recent examples, project case studies and architectural details, this book provide a basis for understanding the nature of the material and the opportunities it offers in new work.

With over 150 color images, this volume provides a concise resource for all those considering terra cotta as a façade system: architects, façade engineers, cladding subcontractors, materials suppliers, developers and prospective clients. With inspiring examples of expressive possibility, this invaluable book will find a home with students and professionals alike interested in making rich, colorful and durable buildings.

Chapter 1 Origins

DOI: 10.4324/9780429057915-2

The origins of architectural terra cotta emerge from the history of earthen construction, as traced by Norman Davey in A History of Building Materials. Babylonians and Assyrians applied burned clay as cladding, using wall sections in which the weathering surface was consciously differentiated from the body of the building underneath. Beyond the functional advantages, the distinct outer shell provided a place to present symbols, narratives and ornament.

The famous Isthar gate in Babylon, built by Nebuchadnezzer II (604–562 bce) … was faced with kiln baked bricks bedded in bitumen.¹

Each large city usually had its tower, or ziggurat, on which a shrine was erected, and these were of sun baked brick cased in burnt brick, but one at the tower of Erech had its outer surface protected by thousands of pieces of pottery hammered into the brickwork while it was slightly plastic.²

Both the Babylonians and the Assyrians from the ninth to the sixth century [bce] made patterned bricks and wall tiles with colored glazes, and the Palace of the Achaemenid Kings of Persia at Susa had brick friezes decorated in relief in this manner. A particularly fine example is the “Frieze of the Archers” from Susa, dating from about 500 [bce], depicting figures of the royal bodyguard, now in the Louvre, Paris.³

1 Davey, p. 67. 2 Davey, pp. 23–4. 3 Davey, p. 69.

Architectural terra cotta has physical and functional antecedents in ancient times, yet the name itself is surprisingly recent. “Since Antiquity, builders have used fired clay for architectural features, but the term ‘terra cotta’ (baked earth) dates from the eighteenth-century revival of the medium.”⁴ In 1886, at the apex of that revival, James Doulton defined architectural terra cotta as follows:

That class of ware used in the construction of buildings which is more or less ornamental and of a higher class than ordinary bricks, demanding more care in the choice and manipulation of the clay and much harder firing, and being, consequently, more durable and better fitted for moulded and modelled work.⁵

Terra cotta is distinguished from other clay components through its architectural application, more than purely technical differences. At the time of Doulton’s definition there were surely tall, industrial chimneys built of bricks with greater compressive strengths than those needed for claddings.⁶

Terra cotta is a process as well as a product. It is defined by a combination of factors that only begins with the clay. Operational definitions of clay describe naturally occurring, ultra-fine soils that are plastic and cohesive when damp, leather hard when dried, and a brittle but rock-like mass when fired. A summary of clay origins, molecular structure and related behaviors can be found in Earth, Brick and Terra Cotta, edited by members of Historic England’s Building Conservation and Research Team.⁷

4 Simpson, p. 129. 5 As quoted in Stratton, p. 13. 6 Lehmann, 2020. 7 Henry et al.

Clays are created by chemical decomposition of certain bedrock minerals due to the actions of weathering and hydrothermal and biological processes, and physical disintegration. … Some deposits remain close to the bedrock from which they formed (primary or residual deposits). Others have been carried considerable distances by water, wind or glacial action (secondary or transported deposits). In the course of transportation, soil particles may become segregated according to size, and be deposited in separate layers or beds, as gravel (larger than 2mm); sand (2mm - 0.063mm); silt (0.063mm – 0.002mm); or clay (less than 0.002mm).

Clay minerals are inorganic crystalline substances with an atomic structure consisting of sheets of silica and alumina (or sometimes magnesia) arranged in parallel layers. The stacking arrangement of these layers, and the ions and water molecules that link them, define the various clay minerals, and influence their respective properties and behavior.

Clay-rich soils remain plastic and deformable over a wide range of moisture contents. The upper and lower limits of this behavior are defined as the liquid limit and plastic limit respectively. At moisture contents above the liquid limit, clay particles are suspended in free water and the mixture will flow. But when the moisture content falls below the plastic limit, the particles bind together, and the material becomes brittle.⁸

Brick, through most of history, has been known as a regional material. Clay of sufficient quality was found close to the point of end use, giving bricks distinctive local characteristics, particularly color. Terra cotta companies also established plants at particular clay sources. A cluster of villages south of Florence, Italy, referred to as Impruneta, are known for local clay that delivers products of exceptional frost resistance and durability, ranging from garden pots to architectural components.

In the eastern United States, a consensus emerged that it was less expensive to ship raw material in bulk than it was carefully packed finished pieces. The markets in Chicago and New York were large enough to sustain production facilities close to the construction sites. Whereas forty-eight major American firms once produced terra cotta for building façades, there are now only two remaining: Gladding McBean in Lincoln, California and Boston Valley in Orchard Park, New York. Gladding McBean was established in a rural area north of Sacramento where a bank of pure white kaolin clay had been discovered by accident while local officials were straightening a road. The founders built a large facility that shipped products up and down the West Coast. Boston Valley is supplied with clay from Ohio, long a quality source in the United States. Finished products are shipped far and wide, but significant demand comes from New York City. Terra cotta uses higher grades of clay, in far less quantity than brick production. It is also a higher value end product, allowing the shipping costs to be more readily absorbed into the price.

Contemporary architectural terra cotta moves about within an international marketplace. Top quality, custom work comes from a limited number of production sites. The site chosen or assigned to fill a particular order has to do with the skills and capacities at that plant more than the properties of local clay. The needs of a factory might be filled with clays from a number of suppliers, both near and far.

Clay is mined, usually in open pits. Traditionally, it was stockpiled and allowed to weather for up to a year before use. This was understood to break down the clay lumps in size and allow chemical conversions to reduce the level of common impurities. Modern-day suppliers use mechanical processes to prepare clay without extended weathering.⁹

8 Henry et al., pp. 6–7. 9 Prudon, p. 63.

Clay suppliers may prepare materials for both brick and terra cotta producers, using different techniques. In wet preparation, roller grinders crush the clay and impurities to a specified size. Water is added during the process and the clays are stored for a few weeks so that the moisture levels are evenly distributed. In dry preparation, mechanical grinding occurs without water. Hot air is added and carries the finely ground particles away into storage. Grinding principally adjusts the particle size of the aggregates and impurities in the mix, since the clay is already very fine. Impurities must be small enough to have no negative effects on the end product.¹⁰

There are three major clay types used for terra cotta: china clay (pure kaolinite), ball clay (kaolinite with quartz and mica), marls and fireclay (more quartz than kaolinite, plus other metal oxides and impurities like iron).¹¹ These differ as to plasticity, shrinkage and firing temperature. The history of terra cotta companies records a range of preferences. In ceramics as a craft, clay types have great importance to the artisan. For commercial production of architectural components, the selection criteria are much more pragmatic. Clays must deliver adequate structural strength at reasonable firing temperatures, since energy is a huge component of production cost. Low water absorption and limited impurities are important. Beyond these, the best clay is the one best suited to the processes that will be used by a given company. There must be a secure and consistent supply so that recipes can be reproduced reliably over time. This relates to both appearance and performance. If a large-scale project is built in phases, it must be possible to duplicate the outcome several years later. As a leading producer, NBK Architectural Terra Cotta has plants in Germany, Portugal and China that can all run the same clay. In scheduling production, it may be advantageous to assign various components of a large project to different plants.¹²

Clay enters the production process as bulk dry powders. Mixtures can then be determined by weight for better quality control. Clay alone develops high strengths, but with shrinkage of 12–14%. At this level, formed pieces would be hard to dry and fire without tension cracks. Instead, clay is mixed with “grog,” which is previously fired scrap material that has been ground to particles 1.5 mm (0.06 in) in diameter. Mixing in 30–35% grog reduces shrinkage to 7–8%.¹³

10 Lehmann, 2020. 11 Henry et al., p. 640. 12 Lehmann, 2020. 13 Lehmann, 2020.

The ratios of clays, grog and additives for color change define a particular recipe, coupled with the amount of water to be used. Computer control of the quantities reproduces clay batches with the workability and shrinkage rate that has been anticipated in the design of a component. Without computer control, test samples from a batch have to be dried and fired to confirm the shrinkage. Heavy duty mixers can produce a ton of plastic material in a batch. In the past, mixing might have taken an hour, but now it can be completed in 4–5 minutes. The results are ready to move on to the forming process.

Once clay products are formed, they must be dried carefully. While that process is not technically complex, the pattern and rate of drying controls shrinkage movements. Drying of large pieces is hardest to control. Since they dry from the edge to the center, shrinkage will introduce tension stresses near the perimeter at a time when the green clay has little strength to resist cracking.

The firing process has distinct phases that begin with evaporation of free water from the pores, resulting in more shrinkage. At a higher temperature level, carbonaceous materials are burned out if there is oxygen available to complete the process.

When clay is heated to a temperature above 600°C it undergoes irreversible “ceramic change” when water which forms part of the crystal structure is driven off. … At temperatures above 800°C, the strength and durability of the clay body increase progressively as particles become welded together with glassy material (“vitrification”) and new minerals form and recrystallize. Important among these changes, for the development of mechanical strength, is the formation of the mineral “mullite”, which begins at temperatures between 950°C and 1050°C.¹⁴

The temperature ranges quoted depend on the composition of the materials. Secondary minerals present or added to the clay act as fluxes to influence the temperature at w...

Table of contents

• Cover
• Half Title Page
• Title Page
• Copyright Page
• Table of Contents
• Preface
• Acknowledgments
• Introduction
• 1 Origins
• 2 Durability
• 3 Form and Ornament
• 4 Color, Texture and Finish
• 5 High Performance
• 6 Components and Systems
• 7 Intelligent Variation
• 8 Design Imperatives
• 9 Case Studies
• Bibliography
• Index