An addendum. 24% is the minimum level, as suggested, and Ivo's book (Glass Fact File A-Z, Miller's) mentions 65% content for glass used as radiation shielding.
Memo to oneself - always consult Ivo's book first.
May I put this into a correct context?
In UK and EU law there is a legal limit defining "Crystal Glass" known as UK British Standard BS 3828 (1973)
Standard Number BS 3828:1973
Title Specification for crystal glass
Status Confirmed, Current
Publication Date 28 February 1973 http://shop.bsigroup.com/ProductDetail/?pid=000000000000096400
Confirm Date 01 January 2012
You can buy a copy for £45 online from the BSI shop.
In it there are 4 categories of crystal glass (note in the US this does not apply. The term "crystal glass" means any glass that is crystal clear!)
1.Category 1. Full lead crystal containing over 30% lead oxide and meeting a density limit and a light limit.
2. Category 2. Lead crystal which must contain over 24% lead (except German pressed lead (Bleicrystal gepressd., which can contain 18% - I will explain later)
3.Category 3. Crystal glass containing potash, barium, zinc or lead alone or together of at least 10%
4. Category 4. Plain glass or verre sonore.
I need to get the EU paper in front of me to give fuller details, but there is still an outdated legal definition.
Now there are many "lead free" crystal formulae on the mrket which mostly fall into category 3.
We have invented and patented a "non toxic lead free crystal glass", lighter than lead with no arsenic or antimony which "pings" just like lead crystal and can be cut and acid polished!