Old legal documents are usually longer than the scanners. Hence you have to scan the top and bottom of your document separately and join them together.
Scan the images in 300 dpi.
I use the free Microsoft Image Composite Editor for joining the images.
To convert the colour images into Black and White, load the stitched image into GIMP. Use Filters Artistic Photocopy to convert into Black and White. Then use Colors, Threshold, click on Logarithmic Histogram and and adjust the Black triangle to where the histogram goes suddenly up. Manually adjust the number in the left box to tune the results.
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