The preceeding blog discussed what is probably the "granddaddy" of all cleaning challenges - burned-on carbon. But, as those of us who have been in the cleaning business since Noah launched the ark know, there are a bunch of other cleaning challenges that defy modern technology. In general, these challenges fall into a small group of categories. The first category is insoluble contaminants bonded to a part surface. These are materials that don't qualify as "particles" because they are bonded to the part surface by adhesive forces. This category of contaminants includes paints, rubber, plastic, epoxy and silicone compounds like RTV as well as many others. In most cases, these materials are purposely formulated to have excellent adhesion properties and be resistant to chemical and/or mechanical attack. In short, they were never meant to be removed. In addition, many of these materials have resilient physical properties which prevents fracturing them as a viable means of removal. Again, as in removing burned on carbon, these contaminants are often adhered to substrates that are, themselves, susceptible to chemical attack by most of the likely candidate cleaning chemistries. The second category is removing a "contaminant" that is of the same composition as the substrate it is being removed from. Here, the culprit is usually the cohesive forces that bond the contaminant to the substrate. Good examples of this are removing glass particles from ground glass surfaces and removing carbon particles from machined or polished carbon substrates. Mechanical intervention in the form of brushing, high pressure sprays and ultrasonics are beneficial in most of these applications. A word of caution though - - Any cleaning technology that is able to break the cohesive bond between the contaminant and the substrate is probably just short of being able to break down the substrate itself so control is a major issue. "Overkill" is not an option in these applications. Magnetism, although not the same as cohesion presents nearly the same problem. Magnetic particles, even if initially removed, tend to re-attach as quickly as they are removed. The problem of magnetized parts or particles was discussed in a previous blog.