Sentence:  The conveyance of soap to the <WSD>crutchers</WSD> from the kettle may be by pump or by gravity. In the latter case, which is simpler and involves less trouble, the top level of the <WSD>crutchers</WSD> should be below that of the nigre. 
Meaning of the word marked with <WSD> tags:  A vessel in which the ingredients of soap are stirred with a crutch (form of prop).