Around the turn of the 19th century, the culture of the Enlightenment, alongside the emerging needs for modernisation and industrialisation, give rise to a particular interest in technical aspects of construction. Originating in the scientific, mathematical and technical approach to architecture that several theoreticians of the Enlightenment promoted, this becomes stronger in the late 18th century through the unification and modernisation processes by which Germany is evolving from being an aggregate of several backward states and small political entities still based around medieval forms of administration and an agriculture-based economy into a united, modern and industrialised country.
Premises for the development of concrete
As it turns out, the construction of a national, cultural identity is linked to, inter alia, the ascription of a specific character to the German art of building. One peculiar aspect of thischaracter is a prevailing interest in technical matters – considered a kind of German distinctiveness, as opposed to the otherwise dominant aesthetical argumentations concerning ancient Italian architectural styles. The Ober-Bau-Department (Building Department) of Prussia – the state that will lead the German unification process – strongly promotes this German way of approaching architecture from a technical perspective, as is evidenced in the preface to the first issue of the “Sammlung nützlicher Aufsätze und Nachrichten, die Baukunst betreffend,” a periodical edited by the Ober-Bau-Department from 1797 to 1806. The prevailing artistic issues concerning Italian architecture are criticized, notably targeting those treatises that focus on architectural orders.
“…as if raising and decorating a column were of major importance, whereas all other features that the art of building provides to humans, in each of their undertakings, were irrelevant.”
Instead, the German technical attitude is described as the origin of architectural improvements, in opposition to the impressive constructions of the past in celebrated ancient lands that remained in ruins. However, this does not involve a rejection of the ancient art of building. Quite the contrary, ancient constructions inspire interest for both their aesthetical and technical aspects. As such, technical knowledge and building practice are considered on par with art and theory.
“We could be further developed if art and knowledge had sooner and deeper joined together in architecture. If combined, they can achieve a lot, but not if each of them goes its own way. Artists risk weirdness without a scientific guide, while savants lose their capability to reflect when they are robbed of artistic vision and feelings.”
At the same time as these theoretical argumentations are developed, early policies for modernisation and industrialisation suggest a pressing need to improve construction standards. The conditions of buildings, roads, and even entire villages, towns and landscape are to be developed, for which the availability of solid and durable building materials is crucial. This appears particularly critical in the domain of hydraulic engineering, since, at the turn of the 19th century, many works are still made of timber and perishable materials such as bundle, ropes and clay, while the development of manufacturing and trade requires the construction of efficient roads, railways, waterways and ports provided with solid and durable locks, weirs, bridges and embankments.
The use of masonry, which already has a long tradition in the German construction culture, is encouraged in place of timber and perishable materials. Consequently, increasing attention is paid to the suitable materials for producing mortar, since.
Contemporaneously, several German master builders survey building materials and techniques abroad in more developed neighbouring countries, while the scarcity of natural stones in several German regions, together with the cost of transporting them in light of the lack of modern roads, waterways and railways, results in efforts to develop mortar suitable to mould artificial stones, decorations and ordinary objects such as basins and troughs.
The development of German, modern concrete is part of this general renewal process in construction practices, which involves the improvement of building materials already in use, along with the introduction of new ones. This is made possible through an extraordinary combination of studies about lime and mortar, surveys into ancient and recent building materials and techniques, early experimental uses of concrete and attempts at producing moulded artificial stones, decorations and ordinary objects. The earliest phase of this process, approximately from the late 18th century to the end of the 1810s, is still evolving in the context of the 18th century building culture. Studies and surveys into lime and mortar principally analyse the state-of-the-art knowledge, while some early rudimentary kinds of concrete and mortar, used to mould artificial stones and objects, are produced with typical materials of the 18th century.
Between alchemical heritage and scientific achievements
Early nineteenth-century knowledge about lime originates from ancient Roman building practices, which the eighteenth-century passion for archaeology pushed to study, and from scientific findings about the nature and properties of lime made possible by the young science of chemistry – despite the survival of some alchemical beliefs. Lime is thought to be a substance produced from raw materials containing a combination of carbonic acid, crystallized water and lime earth, which is also called carbonated lime. Limestone is the most frequently used raw material in producing lime for constructions. Fired in kilns, it is thought to lose water and carbonic acid, while lime earth combines with a supposed existing substance of heat and forms quick lime. It is believed that this tends to build its original state up again, combining with the same kinds of substances that are lost during burning. For this reason, quick-lime is supposed to combine with water, release the substance of heat and produce the so-called slaked lime, which is then thought to combine with carbonic acid from the air to provoke the phenomenon of hardening that changes it into a kind of reconstituted limestone.
Most German master builders, architects, engineers and even chemists from the early 19th century consider such chemical explications reliable. In an extended survey about limestone, lime and mortar, which is published in several issues in the “Sammlung nützlicher Aufsätze und Nachrichten, die Baukunst betreffend,” between 1799 and 1800.
“…just the inclination [of lime] to combine with the water that has been lost during burning, and the capability to fix it as crystallized water, make the use of lime for the production of mortar possible.”
– Paul Ludwig Simon (1771–1815) –
“Through burning, lime earth undergoes an essential transformation. It is set free from carbonic acid and water, with which it was initially combined by nature, while the substance of heat pervades it. Later on, during the slaking process, the substance of heat parts with lime, which, instead, combines with water, due to great affinity between lime and water. Slaked lime has the property of absorbing carbonic acid from the air, adhering to surfaces of smooth bodies, and hardening to the point of becoming like a stone.”
– David Gilly (1748–1808) –
Eytelwein essentially shares the same belief, although with a slight difference. He believes in the affinity between water contained in slaked lime and carbonic acid, rather than in the affinity between lime and carbonic acid. “Pure lime earth,” he writes, “strongly tends to combine with water, but it has a quite low tendency to combine with carbonic acid. Instead, water has a strong inclination to join with carbonic acid and, since the atmosphere is full of it, water contained in slaked lime first combines with carbonic acid in the air and, only after it is entirely vaporized, carbonic acid and lime combine and produce limestone.”
Despite such minor divergences, convictions about the chemical processes concerning lime are quite coherent with each other and have two important implications in building practices. The former is that lime is mainly considered an aerial binder that hardens only in the presence of air, while the latter concerns the fact that only limestones that contain much carbonated lime are considered appropriate raw materials for producing lime. They are defined “pure limestones” (reine Kalksteine) and are described as particularly white and hard. “Pure raw limestone […] contains no other components than lime earth, carbonic acid and water,” Eytelwein asserts in this regard, and Gilly further- more maintains: “the more limestone is hard, the more it gives good lime […]; therefore marble gives the best, hardest, and finest lime.” There are quite pure limestones in quarries near the village of Rüdersdorf, in East Berlin. In his studies about lime, Simon analyses Rüdersdorf limestones and establishes that the carbonated lime can reach 97% of the entire composition.
Carbonated lime is also the main component of shells, which are therefore used to produce a kind of lime called Muschelkalk. The practice of producing lime from shells is very common in the Netherlands. In Germany, it is known through accounts given in some German technical literature of the second half of the 18th century. Engineer Friedrich August Alexander Eversmann (1759–1837) describes the Dutch production of Muschelkalk in a book published in 1792. Economy professor Johann Beckmann (1739–1811) also writes about Dutch Muschelkalk in his treatise Anlteitung zur Technologie, which is published in six editions between 1777 and 1809. Following such observations, Muschelkalk is produced in German Friesland, which is located near the Netherlands. It is documented that at least two lime kilns are producing lime from shells in the 1820s, the Roßmann und Kipp in the village of Stade, and the Brunkhorst und Westphalen in Buxtehude near Hamburg.
Despite all theoretical beliefs about chemical processes pertaining to lime, practice shows that limestone also contain other substances besides carbonated lime. The most common ones are silica, clay and metal oxides. Since they are not supposed to have any chemical role in lime transformations, these substances are considered as impurities. Too much clay is even considered harmful, since clay is thought to glaze around lime particles, hindering the combination of lime with water. In such cases lime is called todgebrannter Kalk (dead-fired lime). These convictions notwithstanding, some kinds of limestone containing clay – called Märgel – are used to burn lime in regions where pure limestone is not available, as is the case in northern Brandenburg and Pomerania. This kind of lime is called Märgelkalk and is considered a low-quality material.