Soldering problem

During the St Albans DMES Saturday zoom chat session one of the members, and experienced modeller, was having problems with soldering a casting. It appeared to be brass by both weight and colour but he couldn't get solder to take to it despite trying all the wide variations of well known solders and scraping tricks as used for aluminium, and various fluxes and temperatures, and of course multiple variations in cleaning preparation.
One thing he noted was that it turned black during the heating process. Before you say 'yeah copper goes black' I repeat he is an experienced modeller so has encountered all the standard stuff.
It is just 'decorative' so no need to be an exotic or even uncommon material for strength or other reasons.
Al bronze sprung to mind but it normally works with a strong flux and or scraping plus why would it be used for a run of the mill castng.

I suspect that this casting is made from 'spelter', an indeterminate term which means different things to different people in different times, but loosely interpreted as 'brass trimmings, scrap and swarf' chucked unceremoniously in a crucible and heated to melting point....

....Unfortunately, during this melting of brass junk there was plenty of oppportunity to add contaminants that make use of modern solders problematic....antimony, is common and can have detrimental affects, as can a variety of base metals and stuff like mercury and oily muck......

I have come up against 'spelter' clock frames and cases, notably those extravagant looking french clocks with partial greek columns and trophies of laurel leaves etc....vile in both appearance and content! It's common to find that they can't be repaired except by drilling and plugging. Sometimes the 'metal' will not even allow threading it's so poor in quality..

The solder will only wet onto the brass if the surface is clean ie free of carbon, sand,etc and oxides. You have dismissed surface contamination which leaves oxides as the problem.

The function of the flux is to remove ALL oxides formed prior to and during the heating cycle. There is no universal flux, and I don't understand what is meant by a "strong flux". Al bronze can only be soldered or brazed with the right flux!

Not knowing what is in the brass, it's one guess after another til you get it right.

More info is needed

Keith