DEVELOPMENT OF A DETERGENT TEST--PART I W. B. SMITH, B.Sc., and A. TAYLOR, M.P.S.* The approach to the general problem o[ devising usehal laboratory service tests is described using as an example the development ot a test •or the deter- gency o[ dishwashing products. The economy o[ e[[ort and additional value o[ the results obtained by the use o[ a Factorial Design [or the tests over those obtained [rom the Classical Method o[ examining the e[[ect o[ changing each variable singly is emphasised. The results o• the artificial soil test developed agree with those •ound with a natural soil test. !n•ormation is obtained regarding the best method o• interpretation o[ the data, the repeat- abili• o• the test, the precision o[ different operators and the effect o• uncon- Bollable variables. INTRODUCTION TI4• ULTm^T• test of a detergent--as of many things--is its performance under actual use conditions. Though laboratory determinations of what are considered the prime requisites can be utilised as valuable screening tests, the final evaluations (or at least the later stages of evaluation) are normally made by performing actual detergency operations ' the detergent under test and a standard product are successively or simultaneously used for washing soiled articles under closely controlled conditions. In specifying a useful test procedure of this type it is necessary to deter- mine what variables require control, to what extent they need to be con- trolled, and what are the optimum values for the variables. In this connec- tion it is necessary to ensure that the conditions selected really provide the required objective test of the detergent, which implies that slightly different levels of control (for example, using natural instead of artlficial soils)' will give essentially the same comparison of the sample with the standard. Finally, it is desirable to determine the precision of the test procedure:--its repeatability and reproducibility--in'order that useful and valid inferences may be drawn from the results. This article describes the experiments carried out on these lines on a method'•'C•f evaluating detergents for dish washing, and is offered in the hope that the examples will prove instructive and useful for persons engaged on analago. us problems. THE TEST PROCEDURE The quantity to be determined is the number of plates that can be washed under typical domestic conditions with given concentrations of the detergents under test and the procedure consists of washing soiled dinner plates one at a time until the detergent solution is "finished." * IV[archon Products, Ltd., Whitehaven, C[•mberland. 96

DEVELOPMENT OF A DETERGENT TEST--I This dishwashing test, it may be well to point out, is not the only impor- tant factor in evaluating a detergent, for the product must satisfy other ., conditions, such as, it must not be harmful to the skin, cause corrosion or tarnishing of metals, or staining of plastics. Also, we are here not so much interested in the efficiency of a detergent as in its capacity for a certain amount of work. To investigate the former one would probably use glass surface and instrumental methods of measuring the trace of foodstuff, scum, or film that remains on the washed article. The argument concerning the respective merits of natural and artificial soils seems to occur in practically all tests of detergency. Generally, both types of soil have to be used, artificial methods of soiling being needed when insufficient naturally soiled test material is available and also tests must :• include the use of natural soils to ensure that most of the conditions encountered in practice are represented. An artificial soil (designated as type Q) was used in the experiments detailed below. It was prepared by melting 1,500 grams of a proprietary ß cooking fat--"Trex"--at 50 ø C., adding 60 grams oleic acid, 3 grams oil- soluble blue dye, and mixing in 1,440 grams flour. Stirring was continued during cooling. Shortly before a group of tests, the soil is applied to the plates (white dinner plates of 25 cm. diam.) by the operator who is to wash them. Eight grams of the soil is weighed out on a small square of p.v.c. film, and transferred to each plate, and is then spread into a uniform layer. The soiled plates are stacked in racks, to prevent the soil from one plate contaminating any other, until ready for use--that is, for at least half an hour and no longer than three hours. As it is the use of detergents for domestic dishwashing that we are studying, we use a conventional washbowl (30 centimetres diameter at the water level and 10 centimetres deep from the rim), the plates being washed with a small bristle brush of about 5 cm. diameter. This limits the amount of work that can be exerted in removing the soil and is, therefore, expected to give more reproducible results by different operators than would a dish cloth or dish mop. Nevertheless, differences between different operators are still appreciable and hence detergents are tested in blocks of 2 to 12 samples, all tests in one block being made by the same operator and using plates selected at random from stacks prepared at the same time. The standard conditions are provisionally selected as using 4 litres of water (soft or hard as required by the scheme of testing) with an initial temperature of 47 q- 1 ø C. The plates are at room temperature, and thus the detergent solution will cool as washing proceeds--the troublesome maintenance of a constant temperature is preferably avoided. After adding the detergent (all materials being in solution) the operator uses a wire beater held in the hand to beat up a foam for 60 seconds. Washing of the plates one at a time in a smooth rhythmical manner is then carried out until the 97