J. Soc. Cosmetic Chemists 17 45-56 (1966) ¸ 1966 Society of Cosmetic Chemists of Great Britain The subjective assessment of the consistency of materials in relation to physical measurements G. W. SCOTT BLAIR* A lecture delivered before the Society on 7th October 1965. 1tlr•01•si•--In all traditional, and even in some modern industries, the properties of materials and products are assessed by experts handling them. This is not in itself a bad thing but experts are becoming increasingly difficult to find and it is useful both for the craftsmen themselves and for those working in their absence to be able to compare such subjective assessments with physical measurements and chemical analyses. This paper is concerned with one group of "properties" commonly called "consistency." One must first ask whether the sensations perceived by the experts are themselves quantitatively measurable and, if not, how do we regard the judgements which they are undoubtedly able to make? A second question follows: By what combinations of stresses, strains and times do people assess consistency when they squeeze materials by hand ? Are such combinations dimensionally similar to those which we use to define such physical properties as viscosity and elastic moduli? What mathematical and statistical techniques are required to describe the judgements offered ? Finally, the implications of psychorheology to the practical chemist or physicist working in a traditional industry are discussed. Some years ago, I had the privilege of addressing you on the rheology of pastes, etc. (1) and now I should like to discuss how physical, and particularly rheological properties of materials, as measured by instruments, may be related to the subjective assessment of con- sistency by handling materials. Consistency has been defined as "that property of a material by which it resists permanent change of shape ß . . defined by the complete flow-force relation" but, in fact, the word is often used in an even wider sense. I have argued elsewhere (2) that rheologists require two types of technical terms-those which express precisely in c.g.s. units the properties of materials, such as *National Institute for Research in Dairying, Shinfield, Reading. 45

46 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS viscosity, elastic moduli, relaxation times, etc., and terms which denote general similarities of behaviour, such as consistency, body, etc. If we give these latter terms, which I have called "denotative"* precise physical definitions, we shall have to invent new terms to replace them. It is better to invent new terms for the "connotative" physical properties where these are needed. So we will keep "consistency" as a very general term. It is the purpose of rheology to express the consistency of materials by means of numbers and this can, of course, be done both by subjective assessments and by instrumental tests. The question arises whether we can regard as a true "measurement," numbers which are scored by sub- jective tests, e.g. "very firm = 5, firm = 4, medium = 3, soft = 2, very soft = 1." I think we must accept the word "measurement" in its widest sense, so long as we are careful to appreciate that there is a hierarchy of types of measurement and that certain statistical treatments are applicable only to certain types of measurement. It is also interesting that there is a hierarchy of the senses. An appreciation of the visual arts and of music is always regarded as desirable. Exact science is based almost entirely (some say entirely) on visible pointer readings for which we require only one colour-blind eye. A few scientific measurements, such as electrical resistance, are sometimes made through hearing. It is hard to find exact physical correlates for taste and still harder for smell, and the perfumers and restaurateurs will agree that there is still some prejudice against too great an interest in their products. Aesthetics deals hardly at all with the "feel" of materials. Is there any explanation for this strange parallelism? In our own day, the most intensive studies in this field of measurement theory have been made by S. S. Stevens (3)$, who defined "measurement" as "the assignment of numerals to things so as to represent facts and conventions about them." Although he has later proposed a somewhat more complex classification, his earlier grouping of types of measurement will serve for our present purpose. Stevens originally proposed four types of measure- ment, easily memorized because their initial letters form the French word for "black" (NOIR). Nominal measurements, which some of us would hardly include as measurements at all, represent the arbitrary numbering of people or things *The terms "denotative" and "connotative" have changed their meaning somewhat in the course of time (2). }Stevens has published so many admirable papers on measurement that it is hard to know which to quote. Perhaps the best fairly recent expression of his views is to be found in (4).