Hi Hielke,
a window etch is basically cleaning a small area of the blade (the 'window') and then applying a small amount of etchant (vinegar would be fine) to the cleaned 'window' area, and left for 10 -30 minutes. Reaction times vary due to temperature, strength of etchant etc.
It allows you to check the structure of the blade, without removing all the 'patina' from the blade.
If you are lucky enough to find nice laminations or wootz you can then decide whether to clean and etch the entire blade. If the result is not pleasing, remove all traces of the ethchant and wipe with a salt water/vinegar solution (just the window area) repeat this often and a small rusty area will develop. Then gently clean this area. This is done to 'knock back' the shiny appearance of the window which would otherwise 'stand out'.
Other advice on etchants and etching can be found by using the search facillity on this forum. Hope this helps.
Regards David
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