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Paint sample or chip tests for paint type identification: oil/alkyd vs. latex/acrylic. This article describes a simple procedure for preparing paint samples for reflected light low to high magnification microscopic examination in the paint failure laboratory. A procedure is described for mounting sectioned paint chips on edge for microscopic examination.
This procedure is useful in the determination of paint layers, paint layer thickness measurement, and paint layer interactions. The procedure also permits detection of mold or debris which has been painted over. Two simple chemical tests are described for identification of paint as acrylic or alkyd, useful as a quick, inexpensive alternative to pyrolysis gas chromatography, We include photographs of the procedure for preparing paint chips for cross-section and flat surface analysis. and photos of the results of simple chemical analysis to identify paint samples as acrylic or alkyd paints.
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This article is part of our series: Diagnosing and Preventing Paint Failure on Building Exteriors. Readers should also see Paint Failure Case Photographs-SITE and then Paint Failure Case Photographs-LAB and see PAINT & STAIN GUIDE, EXTERIOR for a guide to the selection and proper application of paints and stains on exterior wood surfaces.
The diagnosis and cure of paint failure on buildings, particularly on wood siding and trim, is quite possible if there is a careful and thorough inspection of the building, its history, its surfaces, and the actual points of paint failure. It is diagnostic to compare the same coating on the same type of surface at different locations on a building and in areas of failed and not-failed paint.
The identification of a paint chip sample as acrylic or alkyd-based is important for art conservationists, but also for more prosaic paint failure investigators who examine buildings.
During investigation of peeling and flaking paint on a costly historic restoration project of an 18th century building we suspected that a contractor had been supplied incompatible paints or that not all of the primer used on the building was the same material, and that the primer was not intended for exterior use. The paint sample above is from that project.
By identifying the type of paint used we made a definite and easy determination that the paint applied was not the paint for which the park service had contracted.
With as stance from two experts, one an experienced paint chemist and the other, an expert art conservator, we here document the procedure for two very simple chemical tests that can often (not always) distinguish between oil/alkyd paints and latex-based paints.
Paint testing laboratories use either chemical methods and/or infra-red spectrometry to identify binders used in paints. Infrared spectroscopy is the most fundamental way to identify almost any organic material.
Pyrolysis GC/MSD is a useful back up method, and has been used in the art conservation field. See Analysis of Modern Paints at References below.
All organic materials have a unique infrared spectrum or 'finger print'. Some paint testing labs (Bodycote) inform us that the following combination of paint testing methods are the most precise in paint identification and can be used to reverse-engineer a paint from a paint sample.
These tests are costly, ranging from $450. to $1000. per test per sample. In contrast, the chemical tests we describe here should cost less than ten percent of those higher fees. However the tests need to be performed by an experienced forensic investigator or microscopist trained in microchemistry.
A Description of Two Simple Chemical Tests to Examine Paint Chips
Chemical tests to examine paint chips or painted surfaces, while less precise and less informative, are very quick, inexpensive, and can be used in the field as well as in the laboratory. Therefore chemicals are mainly used on site and infra-red is used in the lab.
The most common chemical methods for examining paint in the field or for a "quick lab test" are:
Mixed-base paints lead to ambiguous results: This chemical method for paint testing will not always give a definitive answer. For example if a water-based paint is comprised of both alkyd and acrylate components the chemical test will be ambiguous.
However these tests can be unambiguous: if the surface either turns yellow with NaOH or softens with ethanol, and if the converse test of the same sample using the opposite chemical does not produce the key reaction, then the test is reliable.
We performed the tests shown in this document, followed by sending samples of the same paints to an independent paint testing laboratory for advanced testing. The results of this advanced laboratory analysis will also be reported here as a cross-check on our chemical test for acrylic and alkyd paint.
Ethanol Test on Paint Chips Separate Acrylic from Alkyd
Here are photographs showing the effectiveness of the Ethanol test on two paint chips. One paint chip remained brittle and fragile as it was at the outset, unaffected by the ethanol, indicating that the chip was not an Acrylic. We suspected it was an alkyd paint, confirmed by the KOH test.
The second paint chip becomes soft after about 4 minutes in Ethanol, as we demonstrate by curling and rolling the previously fragile brittle chip using our forceps. This confirms that the ethanol-softened paint was an acrylic.
NaOH or KOH Test on Paint Chips to Separate Acrylic, Latex, from Alkyd
Here are photographs showing the effectiveness of the KOH test on two paint chip samples. One turned yellow (indicating Alkyd paint) and the other was unchanged (indicating not alkyd paint, in this case suspected Acrylic). [Photos coming]
Summarizing these tests: We use either chemical methods and/or infra-red spectrometry to identify binders. Chemicals are mainly used on site and infra-red is used in the lab. The most common chemical methods are:
This chemical method is very crude and will not always give a definite answer (for example if a water-based paint comprises of both alkyd and acrylate components), but if the surface either turns yellow with NaOH or softens with ethanol, then you are in business!
Example Paint Investigation Report Distinguishes Between Oil Based Paint or Varnish & Alkyd
The following photographs and comments were contributed by Fred C. Colley, MPH, PhD, Supervisor, Microbiology Department, Pyxis Laboratories. Five samples collected consisted of finish coating from five different locations on the surface of a basketball court at a community center. The paint test samples illustrated below consist of finish coating from different locations on the surface of the basketball court at a community center.
The samples are identical in appearance. Each consists of pale yellow shavings and dust. The object of analysis is standard microscopic and microchemical examination in order to determine if the samples are oil based or acrylic. Finish samples were removed from the surface of the basketball court by scraping with a sharp chisel. Samples were examined using stereoscopic and polarizing light microscopy. Samples were also exposed to 4% NaOH (sodium hydroxide) and 95% ethyl alcohol (ethanol).
Paint Sample Photo #1 - untreated surface finish shavings
Paint Sample Photo #2 - surface shavings in NaOH
Paint Sample Photo #3 - surface shavings in Ethanol.
These tests were performed in December, 2011 by Dr. Fred Colley and Pyxis Laboratories in Portland OR. Contact information for Dr. Colley and Pyxis are at our References & Technical Reviewers section at the end of this article.
Frequently Asked Questions (FAQs) about paint failure forensic laboratory test procedures: chemical tests and microchemistry
Question: what technique can I use to distinguish between acrylic and oil-based varnishes?
I have a client who suspects that a [wood floor in a sports facility] was varnished with acrylic rather than oil based varnish. Can you advise me on techniques for confirming the identity of varnish scrapings. I generally use polarizing light microscopy for small particle id but would like to know your approach. Thank you. [Anon.]
Reply: these microchemical methods provide a quick simple forensic procedure that often is sufficient for paint base type identification
Anon: During some investigative work for the NPS a while back I contacted the chemists at several paint companies for help with defining an alternative method for paint characterization other than the very expensive (try more than $1000./sample) gas chromatography. Worse, not only was that method very costly but when I went to one of the biggest test outfits in North America, they did a horrible and incompetent job. It was evident that at least some of the most costly and sophisticated paint testing labs, their technicians, and procedures have a rather specific focus (such as auto body paint) that may not include expertise on paint analysis for building failures nor for works of art (my areas of interest).
Please take a look at the microchemistry alternative above at Chemical Tests of Failing Paint. There we distinguish between acrylic/latex and oil/alkyd using this method - I invite you to try it and to let me know how you make out or whether or not you have further questions.
Note that although your question referred to oil based varnish, there is virtually no true oil based paint currently sold (that is traditional linseed oil based paints) - they've all been replaced with alkyds. Therefore I think that's what your client wants to look for. If you know otherwise do let me know.
Incidentally, I considered trying to use PLM along with the Baekke method, our standard color chart, and a full set of refractive index liquids to try identifying paint materials. While that approach is rather good at the forensic identification of a very wide range of particles and fibers, without some help from a paint chemist and some specific information not at hand about the precise optical properties of alkyd paint chips and acrylic paint chips, I'm afraid we'd be wasting our time on that approach when making distinctions of chemical constituents.
Finally, when investigating questions about the possible use of a paint product that was not the one anticipated, in addition to forensic investigation of the paint and the building surfaces, it is often most useful to do some old-fashioned detective leg work: obtain copies of the invoices that document where the coating products were purchased as well as just what was bought. Sometimes too we can find leftover paint or coating and primer cans or containers and we also sometimes find in job site photographs an image of the paint cans and their labels.
Questions & answers or comments about paint testing methods, Ethanol test, KOH test, & forensic microscopy and microchemistry for paint type identification - an aid in paint failure analysis.
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Technical Reviewers & References
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