Here is an extended quote from the excellent and highly recommended Whisky: Technology, Production and Marketing (Handbook of Alcoholic Beverages) Edited by Inge Russell, Charles Bamforth, & Graham Stewart

"Whilst filtered whisky is normally a clear, bright product, two forms of flocculation have occasionally been experienced in Scotch whiskies. The first form is known as ‘reversible floc', and can form when whisky is stored for prolonged periods at very cold temperatures (as may be encountered in cold winter transit). The whisky develops a haze, which disappears when the liquid is warmed and shaken. The main congeners detected in reversible floc are the ethyl esters of long chain fatty acids and larger alkyl esters, both detectable by capillary column GC"“MS. As whiskies are produced for distribution around the world to a wide range of climatic conditions, it has been found that reversible floc formation may be minimized by chill filtering whisky prior to bottling. This process reduces the concentrations of reversible floc-forming material, and has no effect on product character.

The second form is known as ‘irreversible floc'. This shows itself as very small, hair-like crystals of calcium oxalate, which slowly form and settle in the whisky when natural low mg/l concentrations of oxalic acid in the whisky react with similarly low concentrations of calcium ions. Oxalic acid may be determined by ion chromatography and calcium by flame atomic absorption spectrophotometry. Irreversible floc formation is eliminated by ensuring that final calcium concentrations are kept to a minimum by demineralizing the water used for final reduction."