גָּפַר (v. גפף) to make thick, tighten. Denom. גָּפְרִית; fr. which
    Pi. - גִּיפֵּר to make water-tight. Part. pass. מְגוּפָּר, f. מְגוּפֶּרֶת water-tight. B. Bath. 97ᵇ; Tosef. ib. VI, 3 מְגוּפָּרוֹת (defective clay vessels) made tight by a lining of sulphur or pitch.
    Hithpa. - הִתְגַּפֵּר to be darkened through sulphur fumes. Sabb. 18ᵃ; Y. ib. I, 4ᵃ top; Tosef. ib. I, 23 מִתְגַּפְּרִין they (the silver vessels) go through the process of sulphuring.