Just guessing here, but "heavy spar" looks like a literal translation of "tungspat", which is "barite" in English. Or "baryta", if you prefer.
I thought I recalled the term "spar paper" from somewhere so looked it up... from the OED:
Heavy Spar
The native sulphate of barium, barytes; also improperly applied to barium carbonate, and sometimes to the sulphate and carbonate of strontia.
1789 A. CRAWFORD in
Med. Commun. II. 353 The muriated barytes..was obtained by the decomposition of the heavy spar.
Ibid. 356 Heavy spar from the lead mines of Derbyshire.
1805-17 R. JAMESON
Char. Min. (ed. 3) 107 Heavy spar, and actyonite afford examples of the hexahedral prism.
1845 ATKINSON in
Proc. Berw. Nat. Club II. No. 13. 137 It is calc-spar, in a setting of heavy-spar or sulphate of baryta.
1885 Encycl. Brit. XIX. 86/2
Baryta White..is prepared by grinding to a fine powder the pure white native sulphate of baryta (heavy spar).
I really need to find something useful to do with my time on these long cold winter evenings - heading for the darkroom in 10 mins...
Bob.