Glycerol mixed esters, the acyl radicals of which comprise those of one or more a : b -ethylenic-a -(alken-1-yl) monofunctional monocarboxylic acids, as hereinafter defined, and of at least one other monofunctional aliphatic monocarboxylic acid of different structure, may be used as synthetic drying oils and may replace the rapidly drying natural fatty oils which are ordinarily formulated in coating compositions. The acid first mentioned above is a polyunsaturated branched chain acid having an ethylenic double bond between the a - and b -carbons, and attached to the a -carbon, an aliphatic side chain having an ethylenic double bond in the position nearest to the main chain; the remainder of the molecule is substantially immaterial provided the acid is "monofunctional" (i.e. contains no group such as hydroxyl, amino or sulphhydryl, which is known to react with the carboxyl group under normal esterification conditions), and contains no acyl or thienyl group attached to the b -carbon atom; other non-reactive substituent radicals may be present, however, and may be saturated or unsaturated and substituted or not by such inert groups as ether, ketone, halogen or sulphide, but preferably there should be present at most, only one terminal methylene group. Examples of such acids are a -vinyl-cinnamic acid prepared by condensing benzaldehyde and crotonic anhydride in the presence of triethylamine, and 2-benzylidene-3-methylbutene - 3 - oic acid, 2 - isopropenyl - 5 - phenylpentadien - 2 : 4 - oic acid, 2-benzylidene-heptadien-3 : 5-oic acid, and a -vinylcrotonic acid which are similarly prepared. The acids of different structure are preferably unsaturated acids, especially those derived from natural drying or semi-drying oil; they may be saturated or unsaturated, straight or branched chain, and substituted or not by other groups or atoms such as ether, ketone, or halogen which do not react under the esterification conditions; specified acids include perilla oil, oiticica oil, chinawood oil, corn oil, cottonseed oil, and coconut oil acids, oleic acid, stearic acid, lauric acid, crotonic acid and sorbic acid. In one method of preparing the mixed esters an a : b -ethylenic-a -(alken-1-yl) monofunctional monocarboxylic acid is reacted with a glycerol partial ester of a different acid. The glycerol partial ester is first prepared by heating glycerol and a fatty oil, in proportions calculated to give the desired degree of alcoholysis and in the presence of an alcoholysis catalyst (e.g. 0.01 to 0.1 per cent litharge based on the oil), an inert gas such as carbon dioxide or nitrogen being passed over and into the mixture; the glycerol partial ester so produced is then esterified with sufficient of the a : b -unsaturated carboxylic acid to esterify the free hydroxyl groups by heating the mixture with a hydrocarbon solvent such as xylene or toluene, removing the vapours of solvent and water of esterification, separating the solvent and returning it in a continuous manner. Esterification catalysts may be present. A viscous solution of the glycerol mixed ester is obtained from which the solvent can be removed, if desired, by distillation or blowing with carbon dioxide. The above process may be carried out in the absence of a solvent. The mixed esters may also be made by reacting the glycerol, simultaneously or successively, in any order, with the a : b -unsaturated acid and other monocarboxylic acid. Alternatively, a glycerol simple ester of either acid can be reacted successively with additional glycerol and the remaining acid; in the alcoholysis step of this process an ester-interchange catalyst such as litharge, sodium hydroxide, or sodium alcoholate may be present. In these processes the acids may be replaced by their esterifiable derivatives such as their anhydrides, acid halides, or esters with alcohols more volatile than glycerol. The preferred products contain the equivalent of 5-35 per cent of a : b -ethylenic-a -(alken-1-yl) carboxylic acid glyceride, especially in the case of the mixed esters of a -vinyl cinnamic acid and drying or semi-drying oil acids. This desired proportion is normally obtained by using the calculated amount of a : b -unsaturated acid, but a mixed ester of similar properties can be obtained by preparing a mixed ester of a a : b -unsaturated acid glyceride content higher than is desired and blending it with the drying oil whose acids are being used (or a different oil if desired) in an amount calculated to give the desired content of a : b -unsaturated acid glyceride; if the initial a : b -unsaturated acid glyceride content is high, say, above 25 per cent, the blending is preferably carried out at elevated temperatures. In example (1), a partial glycerol ester of linseed oil acids (linseed oil diglyceride) prepared by heating linseed oil with glycerol in an inert atmosphere, is heated with a -vinyl cinnamic acid, and toluene; the vapours of toluene and water of esterification are removed and the toluene separated and returned to the reaction mixture; the resultant mixed ester is heated under reduced pressure to remove the solvent, cooled and filtered. With 0.03 per cent cobalt drier, added as a 2 per cent cobalt naphthenate solution, this oil dries to a hard light-coloured tack-free film. Other solvents specified for use in this solution method include cymene, amylbenzene, tetrachlorethane, anisol and cyclohexanone; in general, aromatic hydrocarbons, chlorinated solvents, ethers and ketones may be used. Example (2) describes a varnish prepared by heating the oil of example (1) in a carbon dioxide atmosphere so bodying the oil, stirring the product with a rosin extended phenol-formaldehyde resin ("Amberol") and lime and adding a hydrocarbon solvent and lead and manganese driers. Example (3) describes the preparation of an enamel which comprises milling a mixture of the oil of example (1) with titanium oxide, antimony oxide and mineral spirits, and sufficient cobalt drier is added to give a composition of about 0.03 per cent cobalt. The mixed esters can be formulated into any other desired type of paint, varnish, lacquer or enamel; thus they can be blended with copal, kauri, ester-gum, oil-soluble or rosin-extended phenol-formaldehyde resins, vinyl or urea-formaldehyde resins; cellulose derivatives such as nitrocellulose, cellulose acetate or aceto-propionate and ethyl cellulose; waxes, solvents, pigments and plasticizers; and to particular advantage with fatty oils, especially drying or semi-drying oils, preferably by hot blending. These compositions can be applied to metal, wood, paper, linen, silk, cotton and regenerated cellulose wrapping foil. Specific manufacturers are linoleum patent leather, linoxyn-type materials, coated copper wire, oiled cloth, oiled silk and sandpaper. The mixed esters may also be used in moulding compositions and putties, and modifying agents for urea- and phenol-formaldehyde resins. Specification 569,404 is referred to. The Specification as open to inspection under Sect. 91 comprises also the preparation of mixed esters of other polyhydric alcohols including pentaerythritol, ethylene glycol, diethylene glycol, triethylene glycol, hexamethylene glycol, decamethylene glycol, erythritol, sorbitol, cyclohexyl-1 : 2-dicarbinol and p : p1-di-(2-hydroxyethyl) benzene. The monofunctional monocarboxylic acids of different structure may be aromatic or aliphatic, open or closed chain and, if the latter, monocyclic, polycyclic, homocyclic or heterocyclic; in addition to the aliphatic acids specified above, p-toluic acid, furoic acid, quinolinic acid, a -naphthionic acid, and phenoxyacetic acid are mentioned. The monofunctional a : b -unsaturated acids may contain aromatic or aliphatic open or closed chain radicals and if the latter, homocyclic or heterocyclic, but no further examples are given. The mixed esters may be prepared by another method comprising heating together a polyhydric alcohol, a polyhydric alcohol ester such as a drying or semi-drying oil and sufficient a : b - ethylenic - a - (alken - 1 - yl) monofunctional monocarboxylic acid to esterify the free hydroxyl groups present in this mixture. This subject-matter does not appear in the Specification as accepted.