- 06 February 2006 -
The Eyes Have It
By John Durkee, jdurkee@precisioncleaning.com

This article¹ is about visual assessment of surface cleanliness. It's about using your eyes to evaluate how well your parts have been cleaned.

This is a traditional noncritical method of verification for cleanliness. A chef takes a taste. A sommelier takes a sniff. A sculptor reaches out and touches. A manager of cleaning facilities looks closely.

Figure 1: Demonstration of observation with UV light.

Visual inspection should be the first cleanliness test done in every situation. It's the cheapest. It's completed the most quickly. It involves the most experienced knowledge (yours).

At least three actions should be taken when a plan of visual inspection is implemented:

Have currently produced references available for comparison of output—both acceptable and not acceptable.

A report that something "looks OK" is a waste of time. The report should speak in terms of what others understand—previous performance or standards.

Metrics for visual observation can and should be developed. A set of five should suffice – with two of them being a completely cleaned piece and the other being one not yet cleaned. That leaves three intermediate ones.

Digital photographs of suitable production will provide good dividends.

Observe from more than one perspective. Said another way, adopt the attitude that the item being inspected may not be acceptable and that the inspector's role is to identify how this is so.

The role of inspectors is not to pass goods, it is to identify the ones which fail. Be thorough, but efficient. Please recall the advice of legendary college basketball coach John Wooden: "...be quick, but don't hurry..."

Document something that was observed—even if it was nothing. If it isn't noted, it's not worth doing. Digital cameras with automatic date and time notation and low-cost removable digital storage media should render all objections to doing this moot.

Microscopic examination may be useful—but only after visual inspection without magnification is complete. Inspection for quality control with magnification often wastes resources on evaluation of discontinuities which aren’t defects at this level of cleanliness.

See the forest first, then the trees.

Microscopic examination can be very useful—when a defect or pattern of defects has been identified. Here the magnification is not used for quality control but as an aid to understanding the mechanism of defect formation.

Generally, visual examination should be non-destructive.

Observation with UV ("Black") Light²
This is another form of visual inspection. It is chiefly focused at identifying the presence of hydrocarbon soils on metal or glass (non-polymeric) surfaces.
Many organic³ materials, including polymers, cleaning agents, and soils, will fluoresce when exposed to "black" light.

Figure 2: Simple, low-cost source of black light.

See Figure 1 for a whimsical demonstration, and Figure 2⁴ to see a simple low-cost source of black light.

The energy in UV light stimulates some—but not all—chemicals. A few electrons in their atoms are stimulated after absorption of a photon of UV light. These electrons become excited and are boosted to a higher energy state.
When these electrons "calm down," they return to their home position in the atom’s structure, and visible light is normally (but not always) emitted.
Basically, light of one frequency (below visible) is absorbed and then light of another frequency (visible) is emitted.

Chemicals that fluoresce when radiated with "black" light include: chlorophyll, quinine, eosin (a dye used in medical examinations), "day-glow" paints, blood, urine, semen, Vitamin A and the B vitamins thiamine, niacin, and riboflavin, many inks including those used on postage stamps, common components of paints, fabric and plastics such as stabilizers and antioxidants, and most hydrocarbon-based fluids used in metal finishing operations.

Management of cleanliness via observation under "black" light involves the following steps:

Establish that the soil being used does fluoresce, and that it can be seen by you when illuminated under "black" illumination both neat and when diluted in the cleaning agent.

"Black light" techniques don't have value if the soils don't fluoresce. The observation of no fluorescence has little meaning if the soiled surface is steel with mill scale—which doesn’t fluoresce.

Establish that the native part does not fluoresce—in the same way.

Develop some reference standards. Without this step, managers won't know if absence of fluorescence under black light means the parts are adequately clean, or not. Here parts are wetted in synthetic mixtures of pure soil at known and dilute concentration in cleaning agent, and digital images are made.
These parts should then be processed using the next operating step in the normal manner. The standards are the images and the processing outcomes.

Train staff in use of these reference standards, and in evaluating cleaning quality of actual parts.

The "black" light examination is also non-destructive. Visual examination, with any light source, should be done in an environment at least as clean as is intended for the parts. Examination of parts larger than a coffee cup can be inefficient.

Tools for Cleanliness Tests
Firms and individuals whose interest in parts cleaning is significant enough to read this article should maintain a small kit of useful mechanical tools for assistance in evaluating part cleanliness.

The kit should include at least the following:

• A clean well-illuminated and well-ventilated unobstructed work area not contaminated by airborne dust or particulate;
• a location where packaged samples, records, reference parts, and these tools can be stored without being contaminated;
• a ~15 power loupe (magnifier used to inspect the test piece for visible particles);
• squares of white cloth (used to wipe test pieces for dirt which can be seen). Commercial products include both wet and dry wipers, as well as non-woven fabrics which can be excellent for collecting particles.
  Residual particulate, oil and grease can often be detected by wiping the test piece with a white cloth. Cotton swabs should also be available;
• several probes for assisting in surface inspection. One should be a toothbrush or paintbrush with soft bristles. Another should be a pair of tweezers;
• a can of pressurized air to blow liquids or other material off parts so surfaces can be inspected;
• numerous paper envelopes, plastic bags, aluminum foil for packaging both clean and soiled parts—including labels and a marker. No unidentified sample should be retained;
• a black light to inspect for residual hydrocarbon-based material, and an unlighted place to use it.

The total bill, at Wal-Mart, for this treasure (other than the last item) should not exceed $50⁵.

Cleanliness testing in metal finishing operations doesn't have to require an atomic force microscope or a "chemistry set." Your eyes, your experience, and a few simple tools should be the foundation upon which any surface cleaning system is managed.

References

1. This article is abstracted from material by this author included in Management of Industrial Cleaning Technology and Processes, to be published by Elsevier in the 2nd Q 2006, (ISBN 0-0804-48887).
2. Light just beyond the violet edge of the visible spectrum is called ultraviolet (UV) light. The best UV light sources produce both long-wavelength (300-400 nanometer "black" light) and short-wavelength (less than 300 nanometer) light.
   The appellation "black" means that the light can't be seen by humans. Commonly, "black" light units consist of partially evacuated glass tubes containing a small amount of mercury.
   In a "black" light unit, the glass is tinted to allow the UV light to pass through, but not the visible light. Fluorescent black light lamps can be purchased. When buying them, check the part number stamped on the glass near the end. The part number should end in a "BLB" suffix. The glass should be a dark purple. Expect to spend around $15 or less.
3. Fluorescence was first noted with minerals. In 1852, George Stokes noted fluorescence in the mineral fluorite. Other minerals which fluoresce are diamonds, calcite, gypsum, ruby, talc, opal, agate, quartz, and amber. Fluorescent molecules tend to have rigid structures and delocalized electrons.
4. Image courtesy of Amberica West. The unit shown sells for less than $30.
5. Additional items which can prove valuable, but which are not essential, are: a digital camera with zoom lens, a 20X lighted viewer, an analytical balance capable of weighing to four decimal places if parts weigh less than ~500 grams, a vise to hold parts for inspection of internal sections, an ultrasonic-powered jewelry cleaner, and a computer into which image files and process data can be stored.

John Durkee is a consultant in metal and critical cleaning. He can be contacted at (e-mail) jdurkee@precisioncleaning.com.