Reclaimed slates - to clean or not to clean?

[builder-girl]

Re-roofing 1840 house, re-using original Welsh slates. I am getting conflicting advice about whether to clean them before re-laying. There is very little moss, some lichen and some general dirt. Can anyone give me the correct answer - and the reason if possible - and best method if cleaning is the right thing to do. Thanks

[Perry525]

Welsh slate by chance is good for 400 years, so it should be reused.
Merely washing with water to remove any grit will do.
Make sure you sort them into similar/identical thicknesses before putting them back.
Having their tops close to the batten above will help to prevent wind rattle.
Don't use half slates, always opt for one and a half widths at edges.
You want them to look right.