The technology of parts cleaning is a very sophisticated science today. Most cleaning challenges can be met with the proper combination of equipment, chemistry and technique. In my 45 year involvement with parts cleaning, I have seen many twists and turns along the way. Applications that were considered "impossible" 10 or 20 years ago are now routinely successful. In spite of all the advances, however, there are a few cleaning challenges that still haven't been conquered. For the applications engineer, it is a very difficult thing to say, "I'm sorry, but I don't have a solution." It's a bit like a Doctor telling a patient that he/she has a disease or condition that can not be cured. It's something you never like to do. However, like the Doctors, those in the parts cleaning "practice" still keep trying! Because it's sometimes more valuable to know what you can't do than what you can, I would like to spend a little blog time on cleaning applications that are still without a practical or universal solution. The upcoming comments can be taken in two ways - - For some, they may be seen as an aid in preventing promises that can't be kept. For others, they will provide a list of challenges which, if they can be solved, promise significant noteriety and potential monetary reward since most are associated with real and identified needs with ready customers awaiting solutions. I would like to start off our discussion of cleaning challenges with one that is very specific and has been bugging me, personally, for a long time - - Removing varnish and carbon deposits from pistons in engine remanufacturing operations. Carbon, in itself, is not really a cleaning challenge. In its pure form as a powder or a compressed solid it's pretty much just another particle removal requirement that can easily be solved using a mild alkaline chemistry containing wetting agents and with a little mechanical assist. However, when the same carbon has been deposited along with "varnish" products of combustion in the cylinder of an internal combustion engine the story changes drastically. That stuff is on there to stay! Although there are chemistries that may attack and dissolve the varnish holding the carbon in place, most of them are very strong caustics like sodium hydroxide. Unfortunately, many pistons with carbon and varnish buildup are made of Aluminum or one of its alloys. Caustic chemistries attack aluminum and, therefore, can not be used! A caustic chemistry with sufficient strength to remove carbon and varnish deposits will also dissolve an aluminum piston. In a limited number of cases, the challenge has been nearly met using a mild caustic in combination with ultrasonics with extended exposure times (hours to days). In most instances, this is not a practical solution because of the old "time is money" addage. Replacing the piston or the entire engine is a more cost effective solution. Of course, there are also pistons made of alloys that are not attacked by caustics. These are usually found in larger engines where the potential payback for cleaning may be substantially greater than it is for an automobile engine, for example. So, under the right conditions removing varnish and carbon deposits from pistons may be practical but in the vast majority of cases it remains unsolved with today's cleaning technologies. Upcoming blogs will discuss more cleaning applications that are more dream than reality.