I lived in Cambridge, UK for a while - all the welding suppliers
sell-out of #14's as important solar events came around.
Not tried myself, but #14 is the acknowledged and tested filter.

My thought - 'cos it's glass and thick, you cannot damage it in any
way which compromises its filtration without that being absolutely
obvious by reason of the glass being broken.
Same reason as condom moulds for forming from the natural latex are
made of glass - they are either fully intact or fully broken - nothing
in-between is possible.