Colloids and Clays: The Pre-History of Nanotechnology

By Will Soutter

Topics Covered

Introduction

Nanotechnology is generally seen as a relatively new invention. It is generally accepted to have been officially born as a discipline with Richard Feynman's 1959 lecture, "There's plenty of room at the bottom", and to have really taken off with the invention of scanning probe microscopy (SPM) and atomic force microscopy (AFM) in the early 1980s.

However, colloid chemistry, which deals with the chemical and physical interactions of nanoscale particles, is a much older field - it has been studied and used in the chemical industry extensively since the early 19th century.

Even before colloids, or nanoparticles, were fully understood, they were used in many manufacturing techniques, to produce glass and ceramics with unusual, attractive properties. This article explores some of the ways in which nanotechnology was used long before it was established as a scientific discipline.

Figure 1. Earthenware pottery, which has been made for centuries, relies on the nanoparticulate, or colloidal, nature of naturally occurring clay for its useful properties. Image credit: California State Parks.

Clay Colloids in Pottery

Figure 2. Porcelain is much stronger than earthenware, and has a more polished appearance, due to the differences in colloidal properties of the types of clay they are made from. Image credit: National Gallery of Art.

Clays are one of the most important types of colloidal material available to us. They are still used extensively in industry today, in a range of applications from low-cost fillers in other materials to catalyst supports for chemical processes.

The naturally occurring nanoscale structure gives clay a huge surface area, and the interactions between the colloidal particles, which vary depending on the exact composition of the clay, result in some useful bulk-scale physical properties. Even in its natural form, these properties are what makes clay-rich soil so fertile and desirable for agriculture.

The largest bulk use of clay is in pottery, which has been around for many centuries. The different types and grades of pottery which can be made result from the varying colloidal properties of the various types of clay they are made from.

Earthenware, a porous, course ceramic, it given sufficient plasticity for the moulding and shaping process by the presence of large amounts of "ball clay" - nanoscale clay colloids which contain some organic matter. This material is still used extensively for tiling, ceramic pots and pans, mugs and crockery, and it often glazed to make it waterproof and for decoration.

The colloids in this clay are extremely small, and behave differently to other clay particles when in solution. The particles, like most clay colloids, are platelet-shaped and have negatively-charged flat sections. Unlike most clay particles, however, they also have positively charged edges, which changes the colloid dynamics almost entirely and results in a different solid structure when the porcelain is fired.

Figure 3. Plate-shaped colloids of kaolinite, or china clay, which give porcelain its fine-grained texture and impermeability. Image credit: NASA Goddard Centre.

Gold Nanoparticles in Ruby Glass

Glassmakers have coloured their products for centuries, by using a variety of inorganic materials mixed into the molten glass. The recipes for these colours have often been handed down, carefully preserved through generations.

What medieval glassmakers did not know is that some of their pigments depend on the optical properties of nanoparticles. Gold and silver nanoparticles were created as added to glass to give it attractive colours, and the true nanoparticulate nature of these materials was not discovered until relatively recently.

The most well-known type of nano-pigmented glass is called "ruby glass". Sources differ as to when and where ruby glass was first made, but it was certainly known in parts of modern-day Germany as early as the 17th century. Some examples of ruby glass have also been found in Roman remains from the 4th or 5th century CE.

The recipe for ruby glass involves dissolving very finely powdered gold in aqua regia (a mixture of concentrated nitric and hydrochloric acids), then adding water and a piece of tin. It is thought that the process was discovered accidentally by working with gold solutions in pewter vessels, which are made largely of tin.

The resulting precipitate is a rich purple in colour, known as "Purple of Cassius", which was used as a glaze for pottery, and mixed with glass to make the deep red colour of ruby glass.

Figure 4. Examples of vases containing ruby glass, made in the 19th century. Ruby glass has been manufactured for centuries, although it was only recently that the pigment used was found to contain gold nanoparticles. Image credit: National Park Service.

Conclusions

Nanotechnology is not as young a discipline as we may expect. Whilst recently developed tools such as SPM and AFM have allowed us a better window into the nanoscale world, the unique material properties which are accessible when dealing with these tiny particles and structures have been known about for some time - in some cases for centuries.

References and Further Reading

• "Colloid and Surface Properties of Clays and Related Minerals" - Giese and van Oss, 2002. ISBN: 0-8247-9527-X
• "Metal Nanoparticles" - Max Planck Institute of Colloids and Interfaces
• "Nanoparticle Stained Glass" - University of Wisconsin-Madison