Kallitype is one of many photographic processes in the iron-silver family, along with many others, including Vandyke bronze and Argyrotype. There are some similarities between these processes, but in calotype, the light-sensitive compound is ferric oxalate; in Vandyke and Argyrotype, it is ferric ammonium citrate. Ferric oxalate is superior in several important respects: it allows for darker shadows, i.e., a higher Dmax, and provides extensive contrast control. The difference in Dmax between these processes is not significant, but well-made comparative prints placed side by side typically show a greater richness of shadows in calotype than in Vandyke or Argyrotype. However, the greatest advantage of calotype over ferric ammonium citrate-based processes is its greater contrast control, allowing negatives to be printed with a wider range of densities than in Vandyke or Argyrotype. The basic theory of callitype printing can be found in Sir John Herschel's 1842 paper, "On the Action of the Rays of the Solar Spectrum on Vegetable Colors, and on Some New Photographic Processes." However, it wasn't until 1889 that W. W. J. Nicol patented the first iron-silver process and is generally considered the inventor of callitype. In Nicol's original patent, the print was developed in a bath of silver nitrate. In the early 1890s, he patented several amendments, one of which recommended using silver nitrate in the sensitizer rather than the developer.