By Will Soutter
Clay Colloids in Pottery
Gold Nanoparticles in Ruby
References and Further Reading
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
probe microscopy (SPM) and atomic force microscopy (AFM) in the early
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
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
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.
Porcelain, which is a much finer-grained ceramic, does not require
glazing - it is inherently waterproof, and has a more attractive
appearance. These properties stem from the main constituent of the clay
used for porcelain, called kaolinite, or china clay.
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
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
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.
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