The dots of half-tone blocks were at first of an easily visible size, but as the method of production became better understood it was found that they could be made so small that they were no longer visible as separate dots to the unaided vision, but that the effect produced was really that of a tone-wash. But the very finely grained blocks were difficult to print from, the ink stuck too readily between the dots, where it was not wanted, and the surface of the paper was not even enough to give a clear impression, even if it was calendered and super-calendered. So something had to be done to give the paper a more level surface, and the needed material was found in china clay mixed more or less with barytes. It is laid down in the _Society of Arts Report_ that the amount of this dressing should in no case exceed 10 per cent. in any paper, but modern “art” papers have for a long time much exceeded this fair proportion. From the beginning some kind of dressing has always been found necessary in the manufacture of paper, but nothing so dangerous as china clay has ever been used. The net result is that almost all modern editions-de-luxe, and fine illustrated books generally, either having half-tone plates or colour plates done by the three colour process, consist of little else but thin sheets of china clay supported by the smallest possible skeleton of wood or other cheap fibres. It must be at once granted that the impressions from delicate half-tone blocks made on clay paper are admirable, and it is also true that the printing ink makes the clay insoluble in water wherever it touches it. But the clay has a certain affinity for damp, and books printed on clay paper very readily show damp stains, and if left for any time in a really damp place they are liable to become solid bricks of white mud, quite impossible to repair. If very dry the non-cohesive clay will turn to a white powder. Moreover, this clay-laden paper will not hold binding threads, so again it has to be preserved. A book printed on such material has to be treated in the same way as I have described in the case of wood pulp paper.