At some time in the 1820s Cassell began to operate the City of London Gas Company's failing tar works. This had been set up on Millwall in the early 1820s. The gas company had bought the site from William Pitcher one of their original subscribers who had a building business near the City's Blackfriars Gas Works. He was probably the Northfleet and Millwall shipbuilder, who lived on the road which ran from Blackwall to the Greenwich Ferry' and had considerable land holdings in the area.
The tar works set up by Chartered Gas Company was at Poplar. A number of London gas companies set up specialist works in this period. None lasted very long and they seem to have been passed to specialist contractors. In the case of the City of London Gas Company's tar works the decision to do this was made following a fire and losses through fraud.
Cassell signed a formal agreement for a lease on the works in 1830 working in partnership with a Mr. George Ward. Ward is a common name in wharfaging and chemical manufacture in this period - a younger George Ward was later to be involved in an East Greenwich tar works together with members of the vinegar making Champion family.
The site of the City of London Gas Company's tar works has not been easy to identify but for reasons to be made clear later, it was probably the site which the Survey of London identifies as 'Lowe's Wharf'. In 1828 Cassell expanded and took over the site of an old mast house further north on Millwall, and eleven years later extended this site again. This later became known as Patentia Wharf.
Cassell sometimes seemed to be in financial trouble. 'Cassell being taken to court for not paying tar bills' and ‘Secretary went to Millwall for the purpose of getting a payment from Messrs. Cassell' are two notes among several in this period. Despite this, however, his name appears most frequently in gas company records as a tar purchaser and presumably all this tar was eventually paid for.
In 1843 the City of London Gas Company noticed that the Millwall premises, which Cassell still rented from them, were very dilapidated and there was no business going on there. Letters of administration were taken out against John Henry Cassell. The City of London Gas Co. then began to prepare to lease the site to 'Mr. Blatchford'. 'Blatchford' is perhaps Mr. Blashfield who around this time actually leased the site on which F.D.Morgan had been operating his building materials business. Two younger Mr. Cassells then approached the City of London Gas Company. One of them, Edwin Edward Cassell, said that the partnership with his father was been dissolved. City then leased the Millwall site to him and he stayed there until the 1880s. It is this lease, noted by the Survey of London, which identifies the site as the old City of London Gas Company tar works.