Can I kasher an antique silver plated cake knife?
A silver plated knife can be kashered. The preferable method is to immerse it in boiling water. However, if this is liable to damage the knife, it is sufficient to clean the knife thoroughly with a scrubber.
Sources: The preferable method of kashering is by means of hag’alah in boiling water, out of concern that the knife was used for hot foods, such as a hot pie. However, even if it was used for hot foods, the principle use of a cake knife is certainly for cold foods, and therefore when doing hag’alah is liable to damage the knife, one can rely on the opinion of the Shulchan Aruch (Orach Chaim 451:6), and follow the majority use of the knife (Mishnah Berurah 451:47; Kaf Hachaim 100).
Since the majority use is for cold foods, the method of kashering is to plant the knife into tough ground ten times (Yoreh De’ah 10:3). Using a scrubber is also good for this purpose (Pischei Teshuvah 10:2).