Formalin is generally the preferred fluid for fixation and is widely used. Formalin is often sold as 37-40% aqueous Formaldehyde. Formaldehyde is a gas produced by the oxidation of methyl alcohol, whereas 100% Formalin is a saturated solution of this gas in water. To make a solution of 10% Formalin, nine parts of water are added to one part of 40% (aqueous) Formaldehyde. Therefore, a 10% solution of Formalin is the equivalent of a 4% solution of Formaldehyde. Formalin/Formaldehyde solutions and vapors are hazardous. Avoid contact with skin and eyes, and use only in a well ventilated area or in a fume-hood.