Sentence:  Keeping "bhūjra patra" or banana leaf above the fourfold cloth is essential. Apart from their attributes to help in this chemical process, they prevent the fall of mercury from the paste through the cloth into the bottom of the pot of "dolā yantra". If during <WSD>fomentation</WSD> mercury penetrates through the layer of paste then it will remain above the layer of leaf placed over the cloth, thus permitting the intended chemical action to continue. 
Meaning of the word marked with <WSD> tags:  The act of fomenting; the application of warm, soft, medicinal substances, as for the purpose of easing pain by relaxing the skin, or of discussing (dispersing) tumours.