Reference for Chullin 6:7
בַּמֶּה מְכַסִּין, וּבַמָּה אֵין מְכַסִּין. מְכַסִּין בְּזֶבֶל הַדַּק, וּבְחֹל הַדַּק, בְּסִיד, וּבְחַרְסִית, וּבִלְבֵנָה וּבִמְגוּפָה שֶׁכְּתָשָׁן. אֲבָל אֵין מְכַסִּין לֹא בְזֶבֶל הַגַּס, וְלֹא בְחוֹל הַגַּס, וְלֹא בִלְבֵנָה וּמְגוּפָה שֶׁלֹא כְתָשָׁן, וְלֹא יִכְפֶּה עָלָיו אֶת הַכֶּלִי. כְּלָל אָמַר רַבָּן שִׁמְעוֹן בֶּן גַּמְלִיאֵל, דָּבָר שֶׁמְּגַדֵּל בּוֹ צְמָחִין, מְכַסִּין בּוֹ. וְשֶׁאֵינוֹ מְגַדֵּל צְמָחִין, אֵין מְכַסִּין בּוֹ:
With what substances is it lawful to cover the blood, and with what may it not be covered? It is lawful to cover with pulverised manure, with fine sand, with mortar, with potsherds, with bricks, or with the earthenware cover [or bung] of a barrel, viz. when these substances had been pulverised, but not with unpulverised manure, coarse sand, or brick, or earthenware covering, which had not been pounded. Nor may it be covered by merely placing a vessel over it. Rabbon Simeon ben Gamaliel laid it down as a rule, "That it is lawful to cover with any substance which would sustain vegetation, but not with substances unfit for the growth of plants."