Racemic acid is an old name for an optically inactive or racemic form of tartaric acid. It is an equal mixture of two mirror-image isomers (enantiomers), optically active in opposing directions.
Its sodium-ammonium salt is unusual among racemic mixtures in that during crystallization it can separate out into two kinds of crystals, each composed of one isomer, and whose macroscopic shapes are mirror images of each other. Thus, Louis Pasteur was able to separate the two enantiomers by picking apart the crystals. it was established that the preparation of crystals was not very reproducible. The crystals deformed, but they were large enough to inspect with the naked eye (microscope not required).