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Cleaning a paint brush in white spirit. © Eugene Brennan |
I've been painting my front door, something I've been putting off for a couple of months. Painting requires preparation and then there's all the cleaning up to do afterwards. The trend now is towards the replacement of oil-based paints for wood and metal by water-based formulations. Presumably this is an eco-driven thing, to reduce emissions of volatile organic compounds (VOCs) into the atmosphere or landfills when paint containers are discarded. I'm not convinced water-based paint is as durable as oil-based types, however it does make the cleaning of brushes easier. Cleaning oil paint off brushes requires the use of a solvent such as turpentine, white spirit or kerosene. Here's where the science comes in, but first some background knowledge on the subject of mixtures. When two or more substances are combined together, the result is known as a mixture. There are several different types of mixtures. A solution is a mixture of a soluble solute, e.g. sugar or salt in a solvent, e.g. water. The solvent doesn't have to be water: another example of a solution is light oil or fuel additive in a solvent such as petrol. Atoms in a mixture don't combine chemically, they just closely intermingle, but don't bond with each other. Another type of mixture is a colloid such as milk or paint or a suspension such as muddy water. The only difference between a colloid and a suspension is the size of the solid particles and the time it takes for them to settle out, if they ever do. Mixtures don't have to be combinations of a solid and liquid; they can also consist of two or more solids— such as sugar and salt, or stones and sand. Gasses also combine to form mixtures. So air is a mixture of nitrogen, oxygen, carbon dioxide and small traces of inert gasses such as neon, argon and xenon.
Often in a lab, it's necessary to separate the individual items in a mixture. The processes used are also employed in industry. Filtering is one technique, but that only works for non-solutions. You can't remove salt from salty water for instance by filtering the solution. That's because the salt exists as individual molecules of sodium chloride in the solution and they would just pass through any filter medium. So to extract it, evaporation of the solvent, water in this case, is the separation technique used, leaving the salt behind. If the mixture is a colloid or suspension, the particles are much larger and so filtering can be used. In a lab, this can be done using a funnel and filter paper. On an industrial level, e.g. in a water treatment plant, muddy water is filtered through sand beds. Hence the colloquial term for the Ballymore Water Treatment Plant, the "Filter Beds". Distillation is another separation technique used for example to separate a solution of alcohol and water when making spirits. Alcohol and water boil at different temperatures: 78.5°C, for alcohol, while water boils at 100°C. Heating a mixture of alcohol and water above 78.5°C causes the alcohol to boil off, leaving the water behind. A condenser or "still" is used and cold water circulating in the condenser cools the evaporated alcohol, creating a condensate, which is then collected. Yet another separation technique is decanting. The principle is simple, give a suspension enough time and the suspended particles will settle out. That's what happens in the BWTP and muddy water from the reservoir is given sufficient time for the suspended mud particles to settle out in tanks. The clearer water from the top is pumped off and filtered through sand. That initial treatment removes material that would quickly clock the filter beds. And that's what I do with the white spirit that I use for cleaning my paint brushes: instead of discarding it, I collect it in a 5 litre container and after 6 months or more, all the suspended solids settle out, leaving a reasonably clear solution. Then I can decant off the white spirit as necessary to fill bean tins for washing paint brushes. The decanted white spirit isn't perfectly clear, but good enough for cleaning brushes.
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© Eugene Brennan |