Today, we seek to cement our knowledge with concrete understanding. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

The history of concrete is pretty skimpy, considering how important it is. Most accounts start with the use of gypsum plaster in ancient Egypt. The Greeks used slaked lime, and the Romans knew how to mix lime with sand and volcanic tuff to make a real cement. In fact, the Romans did some pretty fancy concrete work. The next well-known advance was John Smeaton's invention of a water-resistant, or hydraulic, cement while he was rebuilding the Eddystone Lighthouse in 1756. What we call Portland cement was invented by another Englishman in 1824.

A Russian historian, Znachko-Iavorskii, tells a surprising story about concrete and cement. Too many historians of concrete have studied only written documents. That's not where the story is. The concrete itself survives from Roman times right down through the ages. Znachko-Iavorskii has looked at old concrete all over the world and found that it's remarkably variable.

But chiefly he's found so much very good cement and concrete that's been passed over and forgotten. He finds highly water-resistant plasters from the 4th century BC. He finds that egg whites, Cheshire cheese, and sour camel cream were all used in the Middle Ages to make cements water-resistant. He finds a great deal of medieval, and even Roman, concrete that would easily pass today's standards.

He tells us something historians of technology have learned the hard way, and only during the last 50 years. The scribes of kings and emperors didn't write down the means used by craftsmen out behind the castle. Documentation of ancient technology is very minimal. The word technology itself is a modern concept. It literally means the study or lore of technique. Engineering textbooks -- that written lore -- are really very new.

Consequently, an art that is as base, and yet as fundamentally important, as mixing concrete was learned and forgotten a hundred times. Some of the ancient hydraulic cements made from local limes in Kiev, Riga, and St. Petersburg greatly exceed today's strength standards. Yet it's been a practice in those regions to bring in expensive Portland cement, because the potential of native materials has been forgotten.

Only in very recent times have we written down our concrete-making techniques and made it possible to compare, judge, and improve them. By making concrete work into a technology, we've only recently jumped to the point where we can do things with concrete that would've been unthinkable in the ancient world.

I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.

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Znachko-Iavorskii, I.L., New Methods for the Study and Contemporary Aspects of the History of Cementing Materials. Technology and Culture, Vol. 18, No. 1, 1977.