Hallmarked silver ring turned yellow when heated?

I am repairing a silver ring for a friend and when I annealed it it turned a yellowish colour and smoked slightly! It is hallmarked 925 but theres no makers or assay office hallmark anywhere, which worries me greatly. now it has been annealed its is remarkable soft and brittle. I am very cautious about continuing the repair job as I don't want to completely ruin the ring. Could it be foreign silver with different alloys than I'm used to working with or could it be a fraudulent piece?

You must work harden it (light hammering is sufficient in most cases, you can use plastic hammer too),hardening by heat is possible too but then you must have special kiln fitted with pyrometer. You can test it with acidified sodium dichromate solution (20% is OK, add few drops of sulfuric acid, try to avoid any contact with solution or powder-sodium dichromate is carcinogenic compound!), if it is real silver dark red coloured spot must be result. Hope it helps and good luck!