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<p>[QUOTE=&quot;2manybooks, post: 4311570, member: 8267&quot;]The diazo/whiteprint process was used to make copies of maps and architectural drawings, reproducing them in the same size as the original. The original drawing had to be made on a translucent material (something that would let light through). The original was placed in direct contact with the chemically prepared diazo paper, and run through a machine which exposed the sandwich to UV light. The light deactivated the chemicals in the areas where it could penetrate, but the lines of the original drawing blocked light from penetrating in those areas. The diazo copy was then developed - the areas exposed to light becoming white, and the unexposed lines becoming blue.</p><p><br /></p><p>The image could be reversed/mirrored if the translucent original was placed face down on the diazo paper. They may be unfortunate errors.</p><p><br /></p><p>If they are diazo whiteprints, they may quickly fade if exposed to light. Such copies were usually kept in closed files or, in your case, rolled up to protect them from light. They were not considered to be permanent documents.</p><p><a href="https://en-academic.com/dic.nsf/enwiki/629958" target="_blank" class="externalLink ProxyLink" data-proxy-href="https://en-academic.com/dic.nsf/enwiki/629958" rel="nofollow">https://en-academic.com/dic.nsf/enwiki/629958</a></p><p><br /></p><p>The black-line maps you have shown may have been made with a later photographic reproduction technique. They do not appear to be &quot;blueprints&quot;, which would have white lines on a blue paper.[/QUOTE]</p><p><br /></p>

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