Shengyishe's September 12 sulphuric acid technical monitor put the ten-day minus twenty-day spread at 0.00 yuan per tonne.
The calculation is M10 at 1,821.00 yuan minus M20 at 1,821.00 yuan, with both moving averages converging on the session.
Shengyishe said the M10 moving average crossed above M20, with the spread turning from negative to positive and classifying the move as an upward-start signal.
The golden cross is a lagging technical indicator and can fire after a sharp spot sell-off has already occurred.
SMM's September 11 sulphuric acid weekly review reported the China copper smelting acid index at 1,418 yuan per tonne, down 7.9 percent week over week.
That decline was the largest weekly drop of the year in SMM's index and extended a ten-session run of weakness in spot acid markets.
Regional cuts centered on the Yangtze corridor, where Daye Nonferrous launched a second sharp cut in September after an 85 yuan reduction on September 5 and another 200 yuan cut on September 11.
SMM said weak downstream demand fully offset supply-side reductions, including maintenance at a large Henan smelter since August 20 with cumulative acid impact near 50,000 tonnes.
In the same weekly account SMM flagged domestic sulfur diverging from acid, with Shandong EXW sulfur averaging 8,028.5 yuan, up 0.7 percent week over week.
The technical golden cross and spot acid weakness can coexist when moving averages lag a rapid spot decline that has not yet rolled through the twenty-day window.
For phosphate and fertilizer demand, acid weakness caps sulfur burn even when elemental sulfur spot rebounds domestically.
Alberta recovered sulfur competes into Pacific markets where acid plant economics set the marginal demand for elemental tonnes.
Desk read: September 12's acid golden cross is a technical footnote against a spot market that SMM mapped as still weakening. Watch whether Yangtze acid cuts deepen or whether the crossover marks a pause before autumn phosphate stocking re-engages sulfur demand.