What component of paper emulsion is responsible for toning effects?

Is it the ratio or amount of chloride to bromide? Or is it something in addtion?
I am curious as to why some papers like Forte tone so readily and others like Seagull do not.

I guess an adjunct to the original question would be why do some papers respond much more delicately to bleaching (at least in my experience) then others?

Toning works by converting the metallic silver in the print to something else. The something else is silver sulfide and selenide in selenium toning, silver sulfide in traditional sepia toning, or a thin coating of gold metal in gold toning. A lot about the tone depends on the physical state of the metal you start with and of the intermediate bleached compound produced by the ferricyanide, thiosulfate, and (or) thiocyanate in the toner. Very small grains are converted more easily and more completely. Also, a big, thick grain appears darker and more opaque, even if it is completely converted. Typically, warm toned papers have smaller grains and tone more easily. It's not so much whether you started with chloride or bromide, but what sort of silver grains develop that counts. This grain structure is largely controlled by the way the emulsion is made. Other properties of the paper and coating may also have some effect, but I don't know anything about that.

Depends totally on the type of toner.

If it's a direct toner then yes the balance of chloride to bromide is important, with an indirect toner using a bleach bath it's less relevant.

Again with bleaching chloro-bromide papers will have far more versatility, however it's extremely personal as to how far ypu go with toning.

Ian