Conflicting data may be obtained from accelerated laboratory tests vs. long-term exterior exposures when determining the fungal resistance of organic paint films. Similar imbroglio has resulted from other accelerated testing of paint failure, i.e., salt fog testing for corrosion, adhesion on galvanized steel, and UV light resistance of paints and plastics. This article discusses the trials and tribulations of one individual who realized the almost complete reversal of data from accelerated tests vs. long-term exterior exposure for determining fungal resistance of paint films. He broke from the traditions of a typical microbiologist who depended on accelerated tests and resorted to more realistic long-term exterior exposures and arrived at a prized conclusion. Caution is suggested when extrapolating data from accelerated laboratory tests to form conclusions regarding long-term exposures.

The public is recognizing the roll that microorganisms play in making a house “sick.” People complain they are becoming sick from microbes in homes, factories, police stations, etc. It is timely that the standard accelerated tests involving microorganism control on organic films be reviewed. Such accelerated data may have limited value in extrapolating to both interior and exterior exposures. In a worst-case scenario, interpretation of accelerated data may lead to conclusions and long-term decisions that are completely opposite and erroneous.

It behooves everyone involved with the microorganism problem to be aware of the limitations of data from accelerated laboratory tests. This includes test procedures to screen and evaluate chemicals to control the growth of microbes. Typically, accelerated tests are used to screen chemicals that may control the growth of microorganisms. In doing so, the microbiologist must realize the limitations of projecting data from in-vitro accelerated laboratory testing to control the growth of a large variety of microorganisms to actual in-situ applications.

I, with my limited knowledge of college chemistry and microbiology, was subjected to the limitations of accelerated testing during the tutelage of Dr. Stanley J. Buckman* during his search for a truly long-term effective microbiocide for exterior coatings and plastics.


Dr. Stanley J. Buckman was very familiar with the success of phenylmercury compounds during the 1940s and ‘50s as the best “standard” chemical for controlling the many fungi that grew on paint films. In addition, many companies that manufactured biocides were producing their own brand of phenylmercury products.

Dr. Buckman’s own microbiological laboratories had confirmed the success of phenylmercury compounds in repeated accelerated laboratory tests. These tests evaluated chemicals by placing a given concentration of a chemical within a paint film onto a petri dish along with fungi and, after 10 days, observing the degree of fungal inhibition surrounding that paint film. A large zone of inhibition inferred excellent control. No zone of inhibition indicated no control.

However, there were conflicts. Coatings with phenylmercury compounds performed well with large zones of biological inhibition for 10 days in the accelerated petri dish test in the laboratory, but these same coatings molded readily in a few months upon exterior exposures and subsequently rotted away in several years.

Were fungi becoming adapted to a single microbiocide like the phenylmercury compounds? It was known that certain fungi could tolerate and even utilize such chemicals in their “digestive system.” Dr. Buckman was a leader during the 1940s and early ‘50s in offering two effective microbiocides in a single paint preservative to prevent the adaptation of organisms to a single chemical. Combinations of laboratory-proven microbiocides such as various phenylmercury compounds and various chlorinated phenol compounds did not provide satisfactory durability and long-term preservation against growth on paint films upon exterior exposure.

There were other problems. Mercury products were toxic to handle, they caused sulfide staining of coatings, and mercury was considered an environmental pollutant. Phenol compounds are corrosive, inhibit the drying of oil-based paints, cause fading of colored paints and have an odor. A truly effective biocide was sorely needed.

It was known that zinc oxide did provide some inhibition of fungal growth on paint films upon exterior exposure, i.e., federal specifications for some topcoat oil-based paint films require high concentrations of zinc oxide. However, it was also known that zinc oxide in paint films caused moisture blistering and embrittlement resulting in cracking and flaking of the paint film. Thus, zinc oxide was not a consideration in the search for an effective biocide for a blister-resistant paint film.

Test Procedures for Evaluating Chemicals for Fungal Resistance

Microorganisms are relatively slow to grow on organic films on exterior exposure. Months of exposure time may be required for evidence of growth to occur. Even years may be required for manifestation of film deterioration. In the interest of time, microbiologists resorted to accelerated laboratory test procedures that yield data within 10 days. One such common test is the petri dish test, which provides quick data that is extrapolated to represent long-term exposure results. It is this extrapolation that is in question and is the subject of this article.

The petri dish is simply a flat vessel about 4 inches in diameter with a vertical side about 3/4 inch high. It is supplied with a loose-fitting cover to protect the interior from contamination by airborne organisms. The growth medium in the dish is a sterile nutrient agar on which microorganisms can derive food and moisture to grow. The petri dish is inoculated aseptically, with microbiological tissue (cells, mycelium, spores, etc.) onto the surface of the nutrient agar. After the dish has been inoculated, it is placed in a warm area to incubate. Many fungi will develop substantial mycelial growth at exposures of 78°F for 10 days. These are typical accelerated exposure conditions.

Accelerated Petri Dish Evaluation

The accelerated evaluation of chemicals to inhibit biological growth is determined by placing a spot or tab of a chemical onto the surface of the nutrient agar in a petri dish; this is followed with a flooding of a suspension of biological tissue over the entire nutrient agar surface. One dish is used for each chemical in the evaluation. After a few days of incubation, the degree of biological inhibition is noted. No zone of inhibition indicates no toxicity to that organism and “no control.” A large zone of inhibition infers high toxicity and “excellent control” of that organism with that chemical.

In the paint industry, the test chemical is evaluated at several concentrations in the paint to determine the effectiveness of the chemical to control fungal growth on paint films. Paint samples of each concentration are applied to filter paper and allowed to dry. A square section of the coated filter paper (usually a one-inch square) is used as the tab to be laid on the surface of the nutrient agar prior to inoculating with a suspension of mold tissue. Different species of molds may be used for inoculation; there is a preference for those that grow quickly with a superfluous mycelium. After incubation, the degree of effectiveness of that chemical to control the mold is determined by observing the lowest concentration needed to provide a zone of fungal inhibition surrounding the paint film. A large zone infers high toxicity and excellent control of that mold by that chemical at that concentration.

In my opinion, this method of accelerated petri dish evaluation was used to determine the effectiveness of phenylmercury compounds to control fungal growth, and these compounds then became the successful standard for controlling mold on paint films. There is almost always a large zone of fungal inhibition surrounding the paint film when even low concentrations of phenylmercury compounds are included in the paint film and tested in petri dishes.

The drawn conclusion then was that phenylmercury compounds were excellent microbiocides for paint films. As an example, many years ago one advertisement in a paint journal had a picture of a petri dish showing a large zone of fungal inhibition around the paint film preserved with phenylmercury propionate. The headline of the advertisement read in large print, “The Killer.” However, this “killer” was no better than other phenylmercury compounds in preserving paint films on long-term exterior exposure for the years of exposure necessary for a truly effective microbiocide to preserve an exterior paint film, such as an oil house primer. Not the 10 days in the laboratory, or even two or four years on exterior exposures is long enough. Ideally, an oil-based primer should maintain integrity for the life of the siding on the house.

Reasons for the failure of phenylmercury compounds as a preservative for long-term oil paint films have been reported in the literature. Deterioration, volatilization and leaching by the weather have been mentioned. A series of articles written by Dr. Eric Hoffman (from Australia) are most enlightening.

Experimental Prelude

To demonstrate the existence of conflicting data between petri dish tests and long-term exterior exposures of paint films, Dr. Buckman, during the early 1950s, consulted with a major director of paint test fences to ask his guidance regarding what type of paint might be best for solving the blistering and peeling problems of zinc oxide paint films. The director suggested that a chalk-resistant, talc-extended, long-oil paint would be the most blister resistant and have the best adhesion on wood — if that paint could be made resistant to fungal growth on exterior exposures. That was Dr. Buckman’s challenge! He used that basic formula to test numerous chemicals on exterior test fences in Memphis, TN, an area of warm, humid summers and cold, wet winters.

Long-Term Exterior Exposures

During 1955, Dr. Buckman started his company on the most costly but thorough and ambitious program ever devoted to the test fence screening and evaluation of chemicals in his search for a truly effective biocide. The biocide needed to control the soot-like black fungal growth on the surface of a paint film on exterior exposure and control the literal destruction by fungi upon long-term exterior exposure. He started a “Bulab Paint Test Fence Program.”

Figure 1 shows a general view of the test fences after having been reduced in size and relocated to make space for building construction. The general stature of the Bulab Paint Test Program at that time included the following.

1. Over 58,000 paint test panels with as many as eight different paint systems on each 36-inch panel. Some chemicals did control fungal growth but had other problems, i.e., corrosion, staining, discoloration and loss of adhesion on galvanized steel.

2. A Controlled Temperature and Humidity House for moisture and blister resistance testing. Removing zinc oxide from the basic oil paint provided blister-resistant films. Specially formulated flat alkyd paints performed well even in the hot humid conditions of Houston, TX, when preserved with a good biocide.

3. Hundreds of test houses were painted throughout the United States with as many as 15 different paint systems on each side of a house. Conflicting data existed between house exposures and single off-set panel exposures.

4. Construction of six Eave-Type Test Fence Research Bays wherein 12 ladders of eleven lapped panels (to simulate house construction) both north and south exposure are compared with single off-set panels of 10 different substrates, including various woods, metals and masonry.

5. Developed a “Tropical Chamber Test Exposure” in an effort to develop an accelerated laboratory test for mold resistance to correct for the conflicting data from petri dishes. Modification of this Tropical Chamber has resulted in an ASTM method with some confusing technical problems.

6. Welcomed the technical personnel of paint companies to visit the Bulab Paint Test Fences and to submit samples of paints both with and without Buckman commercial and experimental microbiocides for exposure on Bulab Paint Test Fences. One condition for the exposure of these paints was that the general chemical composition of the test paints be given. Thus, evaluation of microbiocides in many different paint formulations was achieved. This writer made periodic written and photographic reports to the suppliers of the paint samples.

7. Types of panel substrates on the test fences included: cedar, redwood, ponderosa pine, white pine (edge-grain), weathered white pine, bark-side southern pine, pith-side southern pine, Douglas fir plywood, clean hot-rolled steel with millscale, hot-rolled steel with 50% of the millscale rusted off, clean cold-rolled steel, 10 different galvanized steels (from 10 different manufacturers), aluminum, and cement asbestos shingles (new and weathered and some washed to determine the effectiveness of cleaning solutions to remove fungal discoloration. TSP in cleaners increases mold growth).

Interestingly, each type of exposure deserves an in-depth report on the results of long-term exposure. Primary emphasis should be placed on those results that conflicted with accelerated test data. Of most importance are the results of corrosion protection on both hot- and cold-rolled steel and rusty hot-rolled steel. Equally important is the effect of various biocides in paints on various substrates.

In addition to the expansion of the Bulab Paint Test Fences begun during 1955, Dr. Buckman hired several highly trained technical men and four microbiologists during 1956. The technical men were retired technical directors of major paint companies, and their job was to assist in projecting commercial biocides to the various paint companies and raw material manufacturers. The four microbiologists were assigned tasks to determine the problems associated and the specific microorganisms involved for various industries, determine the names of the specific organisms and the extent to which each contributed to the microbiological problem. Their studies have been documented by published articles under the patronage of Buckman Laboratories Inc.

Initially during 1955, my responsibilities were to rate the amount of mold growth and the durability of paint films with dual biocides on exterior exposure, and to assist in the search for new chemicals that might function as anti-biological agents, such as bactericides, insecticides, herbicides, and other fungicides. Based on the physical and the chemical properties and the price, about 500 reasonably prospective biocides were obtained and incorporated into the “alternative chalk-resistant, talc-extended long-oil paint,” exposed on wood panels and observed for varying degrees of mold to develop on these exterior paint films. Usually only six months of exposure were required for significant black soot-type fungal growth to develop on the control without any chemical protection. A second screening study of approximately 10 of the better performing experimental compounds was more thorough and allowed a decision as to which chemical was to be developed commercially. Barium metaborate not only offered mold control but also provided corrosion resistance and markedly inhibited the initiation of chalking.

Subsequently, a patent was received and production was initiated. Commercial barium metaborate was more thoroughly tested in various paint formulations. These paints were compared with other paints made by paint companies on various types of substrates, results of exterior exposure photographed, and reported to companies supplying the test paints and to the technical sales staff of Buckman Laboratories Inc.

Thus, Busan 11-MI, a modified barium metaborate pigment, was introduced to the paint industry. It was not without trials and controversy. Let it suffice here to mention the adverse reception by the microbiologists, who continued to use the accelerated petri dish test as a screening method; the zinc industry, which suffered the loss of zinc oxide sales for use in house paints; the technical men of paint companies, who had depended on the chalk face initiated by zinc oxide with sunlight and moisture to keep the paint film clean of black soot-like spore cluster mold growth; and who depended, and still do to this date, on accelerated laboratory tests for corrosion resistance of paint films. But this is another subject.

Experimental Data

To demonstrate the existence of the conflicting data from accelerated petri dish tests and from long-term exterior panel exposures only three paint exposures were necessary. One paint was treated with phenylmercuric acetate, one contained zinc oxide and one contained Buckman’s new microbiocide, a modified barium metaborate pigment.

As shown in Figure 2, the petri dish test shows a large zone of fungal inhibition surrounding the paint film with phenylmercury acetate. In contrast, the paint film with zinc oxide and the paint film with barium metaborate had no zone of fungal inhibition surrounding those films. These data favoring the phenylmercury preserved paint film were compared with data with the same paints on long-term exterior panel exposure.

On long-term (approximately four years) exposures, as shown in Figure 3, the paint film with phenylmercury acetate had excessive spore-type fungal growth that looks like soot streaking down with the naked eye and the film had poor integrity and no durability. In contrast, the paint film with zinc oxide had only traces of mold growth and had good integrity on this single panel exposure. These results are a complete reversal of those expected from accelerated petri dish tests shown in Figure 2.

A comparison of oil primers, one with phenylmercury acetate and one with barium metaborate, under an unpreserved latex topcoat was made on an exterior exposure panel. After four years’ exposure, as shown in Figure 4, the primer coat with phenylmercury acetate was literally rotting away from under the topcoat, which was hanging in flexible flakes. In contrast, the primer with barium metaborate (Busan 11-MI) under that same unpreserved latex topcoat had excellent integrity and excellent mold resistance thereby providing decorative properties and durability to the total paint system. Again, these results are a reversal of those that would normally be expected from the accelerated petri dish test, as shown in Figure 2.


Factors that may influence the effectiveness of a chemical to control microorganisms on an organic film and the end application of that film include the following.

1. Solubility and leachability of the chemical from each type of organic film.

2. Stability of the chemical to sunlight, long term.

3. The pH of the chemical itself and its effect on the organic film and on the substrate.

4. The pH and the composition of the organic film and their effect on the chemical.

5. The nature and chemical resistance of the contaminating organisms involved, fungi, bacteria, algae, yeasts, and their enzymes.

6. The temperatures to which the preserved organic film will be exposed; including volatility when cutting with a torch.

7. The volatility of the chemial as it decomposes.

8. The concentration, toxicity and the retention of the chemical itself and the resulting components of decomposition to people and the environment.

When evaluating chemicals as microbicides for paint films, an imbroglio exists as a result of conflicting data from accelerated petri dish tests and long-term exterior panel exposures. True, a zone of microbiological inhibition surrounding a paint film is an indication of solubility and toxicity to that test organism for 10 days. However, projecting such data as evidence for long-term effectiveness precludes the influence of environmental factors on the effectiveness of that chemical in situ.


Had Dr. Stanley J. Buckman depended on accelerated petri dish test results for screening potential chemicals as biocides for organic films, a most versatile chemical would not have been commercially developed. His modified barium metaborate pigment, Busan 11-MI, is not only a fungicide, a bacteriostat and enzyme inhibitor for biological control, but it also has superior value as an excellent corrosion inhibitor, an adhesion promoter with reduced loss of metal on galvanized steel and an ultraviolet light stabilizer in paints and plastics.

For more information, contact Lester A. Wienert, Owner, Wienert Mfg. Co., 1601 Amber Place, Rolla, MO 65401.