as has been noted baking soda is generally not enough, you need a stronger base to neutralize the acids (including nitric) in the browning solution.
Also as noted ammonia works as one of the best but if you have problems with it you can also use lye, here's a list of places to obtain ithttp://candleandsoap.about.com/od/coldprocesssoapmaking/a/lyesources.htm, you can use quick lime available from most farm stores, or you can often find WASHING soda in the laundry section of larger grocery stores. Washing soda is a much stronger base than regular baking soda thus works better at neutralizing the acids.
Just add water to either quick lime or washing soda and make a fairly thick, but still somewhat runny slurry.
One other thing is BEFORE using any of the neutralizers wash your piece well with water several times, this dilutes the acids and makes them easier to neutralize. After using also wash well with water.
Of course when using any of the above chemcicals wear a good pair of chemical gloves (available from most hard ware stores these days), and a good full face shield or at leasat some goggles, a rubber apron is not a bad idea.
Now I know that the safety measure sounds like overkill but get splattered, especially on the face and eyes and you can get seriously chemically burned or lose your eyesight - handling even washing soda you'll notice how your fingers feel slick - it's cause these bases are caustic and burn you. A few seconds of precaution can safe you a life time of regrets.
