Since my last post, I’ve been gathering data, analyzing spreadsheets full of numbers, and trying to translate them into useful recommendations for outdoor marble conservation. I will soon reach the end of my time at NCPTT as well as the end of my investigation into six sacrificial anti-graffiti coatings. There have been some surprises along the way, a couple of which I will share here.

Sharpie disappearing act
Ideally, graffiti is removed as soon as it occurs, but in reality it can take days, weeks, or even months for the problem to be addressed. Putting half of my marble samples in the QUV accelerated weathering tester after coating and graffiti application would allow for the graffiti materials to fully crosslink or “cure”. The coatings would also undergo exposure to some of the agents of deterioration present in outdoor settings, such as heat, moisture, and sunlight. I hoped to answer a few questions: are the coatings still effective barriers after this kind of exposure? Are they reversible? Do they change chemically?

Marble samples in QUV artificial weathering tester.

Marble samples in QUV artificial weathering tester.

When I initially designed the experiment, I intended to artificially weather half of the marble samples after graffiti application for 800 hours (about 33 days). Following ASTM Standard G154, the samples would be exposed to 8 hours of UVA exposure at 60°C and 4 hours of condensation at 50°C. Though artificial weathering can never fully predict or reliably replicate how a material will degrade in actual outdoor exposure conditions, it can be a useful tool for producing comparative data. Since my goal was to compare the effectiveness of the coatings after a degree of wear and tear and not to predict how long individual coatings would last, I initially hoped to push the artificial weathering period for as long as possible within the limitations of my ten-week internship.

However, as I rotated my samples in the weathering chamber daily, I noticed that the Sharpie-marked areas were steadily fading and changing color on all of the samples. By 120 hours, those marks were noticeably lighter on all samples, and some, like the samples treated with coating M, were en route to being completely gone.

Samples AQ1 (top) and WQ1 (bottom) after 120 hours of artificial weathering.

Samples AQ1 (top) and WQ1 (bottom) after 120 hours of artificial weathering.

At 285 hours (almost 12 days), contrary to my initial plans, I decided to remove the samples from the QUV weathering chamber. The Sharpie marker had faded so much in some areas, I could see that there soon wouldn’t be much left to clean off. That would have made it difficult to evaluate the effectiveness of the coatings!

Sample MQ1 before artificial weathering.

Sample MQ1 before artificial weathering.

Example of sample (MQ1) after 285 hours of artificial weathering.

Sample (MQ1) after 285 hours of artificial weathering.

Unpredictable angles
Contact angle goniometry is a way to quantify the wettability of any given solid surface. The less wettable a surface, the more easily it repels water. Many anti-graffiti coatings work by decreasing the wettability and surface energy of the coated substrate, to reduce adhesion of pigmented liquids like paint.

A diagram that shows the contact angle and interphase-energy between 3 phases (gas, liquid, solid). Public domain image.

A diagram that shows the contact angle and interphase-energy between 3 phases (gas, liquid, solid). Public domain image accessed 11/18/16 at https://commons.wikimedia.org/wiki/File:Contact_angle.svg

The less wettable a surface, the higher the contact angle (represented by the ?_c in the image). To determine contact angles, I photographed sessile drops in profile on a group of samples before and after coating application. Using these photos, the contact angles were calculated with ImageJ software, a public domain image processing program, using the DropSnake plugin. Surprisingly, the two vegetable polysaccharides made the marble substrate much more wettable after coating application. This unexpected effect did not prevent these two coatings from performing well in the subsequent tests. Even more surprisingly, the substrates retained their increased wettability even after the coatings were ostensibly removed.

Sessile drop one second after deposition on sample AR, before coating application. Left contact angle: 76.1°

Sessile drop one second after deposition on sample AR, before coating application. Left contact angle: 76.1°

Sessile drop one second after deposition on sample AR, after applying one of the vegetable polysaccharide coatings. Left contact angle: 37.7°

Sessile drop one second after deposition on sample AR, after applying one of the vegetable polysaccharide coatings. Left contact angle: 37.7°

 

 

 

 

Stay tuned for the final report summarizing the findings of this study!

National Center for Preservation Technology and Training
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