I have spent considerable time on the blog disclosing and discussing a variety of cleanliness testing methods. A couple of tests that escaped earlier discussion, however, are the “white glove” test and the closely-related “swab” test. These tests are conducted by rubbing or wiping a surface using a white (usually cotton) glove or a cotton swab (like a Q-tip). After wiping, the glove or swab is observed for any evidence of residue having been left on the surface being tested.
Back in the 1950’s, these tests were quite popular and were, indeed, considered the “acid tests” for cleanliness. In fact, they were, and in some cases still are, pretty good indicators of cleanliness in a number of situations but there were, and still are, limitations using this type of test technique.
The first of such problems is access to the contamination. The white glove is useful on flat surfaces and the swab is useful both on flat surfaces and in bores (ID’s, tapped holes, etc). Neither, however, is able to reach into a confined recess (for example) to extract a sample of contaminant. Cleanliness testing using these techniques is limited to surfaces which can be directly accessed.
Another problem is that, for the test to be meaningful, the contaminant must be of a color that will contrast with the white surface of the glove or the swab. A clear or white contaminant may confound the test completely indicating that a surface is free of contaminant when, in fact, it is not!
The test is severely subjective depending on the acuity of the eyesight of the person evaluating the test and, to some degree, on the observing conditions during evaluation. Contaminants, for example, are not as easily seen in the relatively dark confines of a factory environment as they might be in a well-lit laboratory. Even more powerful lighting (sunlight) may reveal discolorations that may not be seen otherwise.
The above reveal instances in which these tests may provide a false indication of cleanliness. There are, however, cases where these tests may falsely indicate a contaminated surface. Some surfaces, notably aluminum, lead and graphite or carbon, will discolor a glove or swab even if they are “completely clean.” These surfaces, when abraded, release particles of the base substrate which, depending on the definition of “clean” may or may not constitute contamination.
These tests are also very sensitive to the pressure and aggressiveness of the person doing the test. Repeated abrasion (even in the same location) or excessive pressure may cause discoloration of the glove or swab while a more gentle approach will not. These things are very difficult to control.
Despite all of the above, the white glove and swab tests are amazingly popular even to this day. As I stated earlier, they are appropriate in specific cases but their limitations must be recognized. As a “quick check” for cleanliness in relatively non-critical applications these simple and inexpensive tests may be all that is required. In critical applications, however, it would be best to employ techniques that are more reliable and accurate providing a quantitative result that can be documented.
– FJF –