What is engineering blue?

Kraft paper is a kind of drawing paper with high transparency, which is widely used to draw various basemaps. It can be understood that the negative (positive) blueprint of a photo is a drawing made of special copy paper through ultraviolet (suitable for sunlight) exposure, ammonia water development and other technological processes. It can be understood that the photo patent application requires that blueprints should not be used, and it can also be understood that originals of drawings are needed and copies should not be used. Blueprint production technology is widely used in all kinds of engineering drawings, especially in large-format engineering drawings such as construction projects, which is irreplaceable by general photocopiers. Large format photocopiers are expensive in equipment and use, and have not completely replaced blueprints at this stage. It should be said that sulfuric acid paper is convenient for construction, which mainly means that the cost of blueprint production is low and it is convenient for large-scale drying. The base map is used for archiving and the blueprint is used for construction.