physics and technology

Wrought or malleable iron has been produced since medieval times but was mainly used for utensils or ornamentation. It was only with the refinement of the puddling process in the 1790s that wrought iron began to have the strength (and fell to a price) where it could be considered as a structural material. For greenhouses this new material had great advantages. Hitherto they had either been stone buildings with large windows or else wood or cast iron framed. Both the latter required thick glazing bars, excluding much light, and only permitted flat surfaces. The new wrought iron was strong even in slender strips and could be shaped without snapping. Suddenly all sorts of new possibilities arose. As far as gardeners were concerned, they wanted curves.

Part of this new requirement was undoubtedly aesthetic; these buildings looked more elegant. Much more important, however, was the proposition put forward by a Scottish scientist, Sir John MacKenzie, in a paper of 1815. He said that the maximum heat transference from the sun through glass occurred when the sun was at a 90 degrees angle. As the sun was at a different point in the sky every day, a flat surface could not be an efficient heat transfer mechanism. Rather the ideal greenhouse would be a sphere.

A sphere, it was agreed, was somewhat impractical. But the curved greenhouse played perfectly to the early nineteenth century obsession with “improvement”, doing things better. Here was the efficient use of solar power with all the attendant savings on coal generated heat.

No one grasped the unfolding possibilities more swiftly than John Claudius Loudon. He had served his horticultural apprenticeship in a Scottish nursery specialising in pineries and so even as a young man was familiar the hothouse challenges. Having made a great deal of money from his garden design business, he decided to go on a grand tour of Europe when travel became possible after Napoleon’s defeat in 1813. He went to St Petersburg. There he was greatly impressed by the massive greenhouses the Russian aristocracy used to produce strawberries in February, but as a good Scottish Presbyterian, he was even more appalled by the extravagance and waste of the whole process.

When he got back to London, the first thing he learnt was that the city broker to whom he had entrusted his fortune (£10,000, perhaps £500,000 today) has lost everything. Understandably Loudon became melancholic, in modern terms depressed. He withdrew to his house in West London building and rebuilding greenhouses in his garden, trying every possible variation. Eventually he was satisfied and sent his optimal plan to the Royal Horticultural Society for their comments. This was the Design Greenhouse of 1818 and was the template which he then licenced to several metal manufacturers. It is not clear, however, whether, despite being the designer and through his journalism and books a tireless promoter of greenhouse gardening, he ever made any money from his design.