Ammonium hydrogen fluoride is a compound formed from ammonium fluoride and hydrogen fluoride, with hydrogen fluoride molecules occupying spaces in the ammonium fluoride crystal lattice. It is the major constituent in glass-etching fluid (though sometimes replaced by potassium hydrogen fluoride), etching occurring due to the fluorination of silicon:
SiO_{2 (s)}+ 4[NH₄][HF₂] _(aq) SiF_{4 (l)} + 4[NH₄]F _(aq) + 2H₂O _(l)

Danger! May Be Fatal If Swallowed Or Inhaled. Affects Respiratory System, Heart, Skeleton, Circulatory System, Central Nervous System And Kidneys. Causes Irritation And Burns To Skin, Eyes And Respiratory Tract. Irritation And Burn Effects May Be Delayed. Harmful If Absorbed Through Skin.

The coated particle according to claim 1, wherein said metal fluoride is selected from the group consisting of lithium fluoride, sodium fluoride, potassium fluoride, rubidium fluoride, cesium fluoride, beryllium fluoride, magnesium fluoride, calcium fluoride, strontium fluoride, barium fluoride, aluminum fluoride, manganese (II) fluoride, iron (II) fluoride, iron (III) fluoride, cobalt (II) fluoride, copper (II) fluoride, zinc fluoride, antimony (III) fluoride, lead (II) fluoride, silver (I) fluoride, cadmium fluoride, tin (II) fluoride, tin (IV) fluoride,