5 CONCLUSION

CARE MUST BE USED in identifying blue pigments in early paint films. In particular, the present paper has shown that 18th-century Prussian blue particle agglomerates, which can be very large or small and angular, may be mistaken for natural ultramarine which they more closely resemble morphologically than they do particles of modern Prussian blue, which is very amorphous in shape. Careful use of analytical techniques, especially polarized light microscopy, can help prevent misidentifications. Blue paint films containing Prussian blue in the form of larger angular particle agglomerates may have been prepared from the pigment made by the earliest described process for its preparation. Such paint films therefore, may be relatively early.

ACKNOWLEDGEMENTS

THE AUTHOR wishes to thank and acknowledge the many hours of expert assistance of Dr. Walter C. McCrone and Mrs. Penelope H. Batcheler, and for the critical insight of Dr. Robert L. Feller and others, for without their support, this project could not have been completed.