TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Method: The magnesium fluoride sol is manufactured by the reaction in ethanol of magnesium acetate and HF, and water and magnesium acetate are present in the solution in a molar ratio of 2:1 to 2.5:1. The molar ratio between fluoride anions and magnesium cations is greater than or equal to 1.5, and less than or equal to 1.95:1. The molar ratio between fluoride anions and magnesium cations in the solution is 1.7:1 to 1.9:1. The HF is introduced into the reaction in the form of a solution of HF in ethanol. The HF is contained in the ethanol/HF solution in a concentration of 5-15 (preferably 10 plus minus 2) wt.%, relative to the total amount of HF and ethanol. The water is introduced into the reaction in the form of magnesium acetate tetrahydrate. At least a part of the magnesium acetate is introduced into the reaction in the form of magnesium acetate tetrahydrate or anhydrous magnesium acetate. The dissolution of magnesium acetate is supported by carbon dioxide. The concentration of magnesium fluoride in the resulting magnesium fluoride sol is 0.35-0.55 (preferably 0.4-0.5) mole/liter. Preferred Sol: The magnesium fluoride sol is free of trifluoroacetic acid. The sol comprises water, ethanol and magnesium fluoride, where the content of magnesium fluoride is 0.4-0.5 mole/liter, and the molar ratio between water and magnesium fluoride is 2:1 to 2.5:1, and which sol is stable when kept at 20 degrees C for at least 1 week (preferably at least 4 weeks and longer). EXAMPLE - Manufacturing of a magnesium fluoride sol was carried out as follows: Anhydrous magnesium acetate (150 g) was suspended in absolute ethanol (850 g) and stirred. Continuously, dry ice was metered into the suspension over 5 hours (100 g of dry ice were added, but a significant amount of it evaporated). The suspension was filtered through paper. A portion of the clear solution was evaporated and dried at 130 degrees C indicating a solid content of 10 wt.%. The solution (100 g) containing around magnesium acetate (10 g) and diluted it with ethanol (30 g). A HF/Ethanol solution (26.7 g) was added, and the solution was stirred. After 4 hours a clear sol was obtained. Magnesium acetate tetrahydrate was dissolved in ethanol (200 g in 1 liter of ethanol). The resulting solution was dried under reduced pressure (30 mbar) at 40 degrees C. After this the water content amounted to 2 mole H2O per mole magnesium. The obtained solid (9 g) was re-suspended in ethanol (100 ml) again and HF-ethanol (10%) (15 g) was added. After one day a clear suspension was obtained. The resulting MgF2 sol, being kept on a shelf, was stable for around two months. Magnesium acetate (dried) (4.71 g) was mixed with magnesium acetate tetrahydrate (4.75 g). The blend was added to ethanol (93 g). After one hour stirring HF-Ethanol (10%) (19.7 g) was added. After one day, a clear sol resulted which proved very stable.