Nor does the forger confine his attentions to the art of the past, but extends them to contemporary work. Constable and Corot were imitated while they were still living; forgeries of Renoir, Degas, Picasso, Matisse, and others are common today; while, among Americans, Winslow Homer and Ryder fabrications circulate freely. Artists are apt to be forgetful as to what they have produced, especially in the case of sketches, and have been known to deny authorship of perfectly genuine work; so that risks of confrontation are not too great. With a contemporary artist recently dead, his work not yet fully known or catalogued, a vogue for collecting him fanned by a skillful entrepreneur, prices not so high as to provoke critical examination, and with not too many genuine examples accessible for comparison, the forger is in velvet. The two main methods of making forgeries, manufacture and misrepresentation, are in practice often combined; but it is convenient to discuss them separately. The simplest type of manufactured forgery is the straight copy, although this has considerable disadvantages. In addition to the necessity of choosing the right materials, imitating the right technique, and giving a proper appearance of age, the risks of confrontation with the original are great in these days of systematic combing of collections, aided by swift and easy travel, by photography and widespread publication. Sometimes, the forger attempts to meet this risk of confrontation by introducing variations into a design, so that the forgery may pass as a version of the original. But even so, comparison of the two is almost inevitable, with the almost equally inevitable exposure of any defects in the copy. It is this risk that makes forgers prefer to copy objects that are types rather than those stamped with the individuality of some particular master. The strictly controlled design and iconography of much Byzantine painting, and its standardized technique, encourages modern repetition; and the putting of one more copy on the market is not in itself likely to arouse suspicion. Similarly, the fact that eighteenth-century Chinese potters paid homage to those of earlier dynasties by making most admirable copies of their work, confuses the situation in favor of the forger. Another advantage (to the forger) of such objects is that many of them can be reproduced by casting. With some knowledge of the materials used for the originals and some skill in giving an appearance of age, such things as Chinese grave figures, Greek or Near Eastern bronzes, and coins can be produced in quantity. Sometimes, indeed, variation in the material of the cast is an aid to deception; as in the case of Renaissance bas-reliefs, when a cast in wax or stucco may, after some manipulation, be passed off as a sketch for a marble original.