Long ago metallurgy and elements available to our industry could not match what we have today. But the legacy or expectations of those days remains. These days we use metals far less dangerous than cadmium and mercury in goldsmithing and gold recovery. Indium and tin and gallium are what’s in the best modern solders.
Why the lesson in solder ingredients? Because you no longer need to go low karat to get low temperature solders for 14kt work. You need not risk dark lines later or brittleness. You as an experienced goldsmith can use plumb solders for every karat without fear of melting the item.