Callot's technical innovations in improving the recipes for etching ground were crucial in allowing etching to rival the detail of engraving, and in the long term spelt the end of artistic engraving. Previously the unreliable nature of the grounds used meant that artists could not risk investing too much effort in an etched plate, as the work might be ruined by leaks in the ground. Equally, multiple stoppings-out, enabling lines etched to different depths by varying lengths of exposure to the acid, had been too risky. Callot led the way in exploiting the new possibilities; most of his etchings are small but full of tiny detail, and he developed a sense of recession in landscape backgrounds in etching with multiple bitings to etch the background more lightly than the foreground. He also used a special etching needle called an "échoppe" to produce swelling lines like those created by the burin in an engraving, and also reinforced the etched lines with a burin after biting; which soon became common practice among etchers. Callot etched a great variety of subjects in over 1400 prints, from grotesques to his tiny but extremely powerful series "Les Grandes Misères de la guerre". Abraham Bosse, a Parisian illustrative etcher popularized Callot's methods in a hugely successful manual for students. His own work is successful in his declared aim of making etchings look like engravings, and is highly evocative of French life at the middle of the century.