Two different methods for the direct application of indigo dye were developed in England in the eighteenth century and remained in use well into the nineteenth century. One of these, known as "china blue", involved iron(II) sulfate. After printing an insoluble form of indigo onto the fabric, the indigo was reduced to "leuco"-indigo in a sequence of baths of ferrous sulfate (with reoxidation to indigo in air between immersions). The china blue process could make sharp designs, but it could not produce the dark hues of other methods.