Oil paints. Fun chemistry experience at home

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Oil paints. Chemical experiments

Entertaining experiments in chemistry

Entertaining experiences at home / Chemistry experiments for children

Any oil paint is a mixture of drying oil with pigments, i.e. with colorants. How to cook them yourself will be described further, and now let's deal with the basis of oil paint - drying oil.

Of course, drying oil (as well as some pigments) can be bought at the store. But for a young chemist, such a decision would be too uninteresting. If we are to experiment, then we must prepare the drying oil ourselves. However, first you need to understand what it is.

Natural drying oil is prepared with vegetable oils. There are many such oils; some of them are able to dry in the air (linen, hemp). Others dry out only partially and are therefore called semi-drying (eg sunflower oil). And some oils do not dry out at all - olive oil from olives, castor oil from castor beans.

But what does the word "dry" mean in relation to oils? There is no solvent in them, and there seems to be nothing to dry. So it is, drying - the term, although well-established, is not entirely accurate. In fact, oils polymerize, their molecules bind to each other, forming a solid insoluble and infusible polymer. But this requires a substance that would sew together relatively small molecules of oil. This substance is oxygen in the air. That is why the film of oil paint applied to the product dries quickly. And the oil left in the jar thickens only from the surface: only it comes into contact with air.

But even drying oils themselves react very slowly with oxygen, and we would have to wait a very long time for the paint to stop smearing. However, this reaction can be greatly accelerated by adding a little catalyst to the oil. Those catalysts that speed up the drying of the oil film are called desiccants. Ordinary drying oil is a mixture of oil (in recent times, increasingly synthetic) with a desiccant.

So, to prepare drying oil, you need to mix the drying oil with a desiccant. You can also buy it in the store, but wouldn't it be better to prepare the desiccant yourself and test its effect by experience?

Many desiccants are salts of organic acids. You can prepare such a salt, quite well known - melted resinate; Resinates are salts of acids that make up rosin.

In a porcelain dish (and if not, then in a metal one), heat about 50 g of rosin. The temperature should be approximately 220-250 ° C, the rosin will melt at this temperature. In this melt in small portions with stirring, enter (with caution, without touching with your hands) 5 g of CaO quicklime. As a result of the reaction, calcium resinate will be obtained.

If, instead of lime, we take 15 g of litharge (lead oxide PbO), ground into a paste with linseed oil, and add it in small portions, then another desiccant-resinate of lead is formed. Litharge can be obtained from lead powder mixed with sodium or potassium nitrate (these are common fertilizers). When the mixture is heated, yellow lead oxide PbO is formed, so that the progress of the reaction can be judged by the color change. When it becomes intense enough, stop heating, cool the mixture and treat it with water. Soluble reaction products will go into solution, and when shaken, light litharge will separate from heavy lead that has not reacted - it will settle to the bottom. Thus, it is necessary to drain the disturbed opaque liquid from the sediment, filter and dry - the litharge is ready.

A mixture of rosin with CaO or PbO must be heated until a homogeneous mass is obtained. Place its drops from time to time on a clean glass, and as soon as the drop is transparent, the heating can be stopped.

Another very common desiccant is manganese oxide MnO.₂. You can easily get it. Prepare two solutions - sodium sulfite Na₂SO₃ and potassium permanganate KMnO₄ (potassium permanganate). When you mix these solutions, a black powder will precipitate. This is the desired oxide. Filter it and air dry without heating.

Whatever siccative you have prepared, the further steps will be the same. Add a desiccant to linseed or hemp oil heated to 150-200 ° C - both are drying. The experience is better if the oil is refined. For 100 parts of oil, take 3-5 parts of a desiccant and stir the mixture until it is completely dissolved. You have prepared drying oil. Let it cool and check the effect of the desiccant.

Apply a thin layer of drying oil on glass or metal, apply the same film of pure oil next to it, without a catalyst. You can easily find out how much the desiccant speeds up the drying of the oil. If you have prepared several desiccants, compare their activity.

To get oil paint, drying oil must be mixed with a pigment, ready-made or home-made. Keep in mind that the pigment must be dried properly, ground and sieved through the finest sieve you can find, or through cheesecloth folded in half.

To such a fine powder, add drying oil drop by drop, stirring and rubbing it each time, until you get a thick homogeneous mass resembling porridge. Rub it thoroughly again and dilute it with the same drying oil so that it takes on the density of ordinary paint. It is possible that your paint will be transparent, and this is not always desirable. To avoid transparency (as experts say, so that the paint is opaque), add some kind of white pigment to the oil. It will not affect the final color of the paint, but will deprive it of transparency.

Even if you do everything carefully, the resulting oil paint will still be, perhaps, a little worse than from the store. But - with your own hands.

Author: Olgin O.M.

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