The silver processing called tula (tipical of German production, from which we take the word), better known in jewelery as niello, is one of the variants used to made overlay decorations during in the early period of fountain pens. This process was typical of European production (and in particular has been used by Kaweco).
The tula is an alloy of silver, lead, copper and sulfur, its color is dark gray, almost black. It's characterized by a low melting point (500 °C) compared to the silver one (about 800 °C). The process involves etchings on the silver overlay, which are then filled by the molten alloy, in order to obtain a decorative contrast between the white of the silver and the dark engravings covered by the tula alloy.
The niello technique is ancient and dates back to the Egyptians. The processing is quite difficult when is made, as in the classical case, to coat silver engraved plates, It becomes even more complex in the case of the coating of a fountain pen, where the cylindrical shape makes it far complex to put the alloy in to the engravings. Because of its rarity, and the great craft value, fountain pens made in tula have a great interest to collectors.