Never judge a book by its cover

By 12/08/2021September 9th, 2021News

Some of us in the post-excavation sections of PCA have recently been working on the report on excavations the company carried out at Ensign Court, off Ensign Street, between Cable Street and The Highway in Tower Hamlets, London, E1. We know, from documentary sources and a previous excavation we conducted over ten years ago on a neighbouring site in Dock Street, that there was a glass house here in the late 17th century, which continued in use into the 18th century. It appears on the famous map of London in 1746 by John Rocque – Glass House Yard and Glass House Hill relate to its location.

John Rocque’s map of 1746 showing Glass House Yard. It is not certain what Glass House Hill was. The Late David Watts, a specialist in the history of post-medieval glass making in London, believed that it might have been a cullet dump, the 17th century equivalent of a bottle bank, for glass to be recycled in the glasshouse.

As far as we can gather, the glass house located on Salt Petre Bank (the earlier name of Dock Street) was owned by the Dallow family. Edward Dallow was one of the leading glass manufacturers in London in the late 17th century and was involved with other notables in trying to rig-the-market in his favour in the manufacture of glass bottles, the ubiquitous and well known thick green glass bottles found on virtually every post-medieval site in the country. A relative, probably his brother, Phillip, became Glass-maker to His Majesty and received in 1689 the patent to manufacture ‘grenado shells of glass’. Hand grenades in thick green glass continued to be made up until the late 19th century (and used with shocking effect in the sieges of the Crimea War).

The classic, ubiquitous, English bottle of the late 17th century. This type of bottle would have been the kind of product made in the glass house we examined.


In 1730 the glass house passed into the hands of Maltis Ryall, who was likely to be the owner at the time of the Rocque map. It would appear that Ryall began to diversify as there are references to the glass house being used to make antiscorbutic waters, a preventative for scurvy. Considering the proximity of the docks this was a clever move.

As for the glass house itself, though, we did not find the main furnace itself but a number of ancillary structures, for the cooling of the vessels and preparation of materials to go into the crucibles, that would have been close by were discovered. Among them is a large quantity of the types of waste we normally encounter when excavating glass works such as pieces of glass from the ends of blowing irons left behind when the vessel has been blown and removed (known as moils), threads of glass that fell to the floor when the glass was drawn from the furnace and droplets of glass that spat out of the crucibles. We also found fragments of the thick-walled clay crucibles covered in glass. These crucibles could contain up to 350kg of glass, enough to make 300 bottles.

These gathering holes would have belched flame and intense heat, as they exposed the glass inside the furnace at temperatures up to 1200 centigrade. Of course, the long blowing irons helped the glass blower to keep away from this intense heat, but it was essential to keep that heat in. Any lost heat could cause issues with the interior of the furnace – and the most disastrous thing that could happen in any glass working operation is the furnace suddenly cooling down. Cracks might appear and the large crucibles inside, if they cracked with a full load of glass, would result in a rapid escape of that glass and a ruined furnace. The operation would have to be shut down, the furnace partially dismantled a new crucible inserted – a most labour intensive and costly task.

The Ensign Court ‘stopper’. The side covered in glass faced the furnace.
The partially vitrified surface on the outside shows the side closest to the furnace when it was set aside.

A common solution, then, was and still is today very simple – ‘stopper-up’ the gathering hole. An elaborate mechanism is not necessary, just a clay tablet thick enough and sturdy enough to resist the intense heat could be placed in front of or into the arched-shaped gathering hole, the official name being simply a ‘stopper’. We were very excited to discover just the very thing during our recent study of the finds from Ensign Court. It is broken in half, but one side that was facing the interior of the furnace is completely vitrified while the other is only partially vitrified – and the location of this vitrification on the right-hand side shows that the tablet was being moved to the left to expose the gathering hole. A small hole that only partially penetrates the tablet was where it could be lifted up and moved on the end of an iron rod (above).

Mark Taylor and David Hill’s first reconstruction of a continental style Roman furnace in 2006, with the cover set aside to the right. Notice also the collars inserted around the gathering hole to reduce the size of the hole to just that required to make the size of vessel.  Every attempt is made to keep the heat from escaping.

Such items are timeless. No doubt Roman and medieval glass workers had similar things – and Mark Taylor and David Hill’s reconstructions of glass working furnaces use their own versions of such things (above). In the middle of the 18th century, the encyclopaedist Diderot recorded these – also in a glass house for making bottles (below).

An image from Diderot’s 18th century encyclopaedia showing a stopper set aside while the blower accesses the glass in the furnace.


When we find the dismantled remains of any type of industrial structure, we can only imagine the intense heat that such structures might have generated. But this small tablet, not a particularly attractive artefact, conjures up the glassblower ordering the cover to be moved, then gathering glass from the crucible, and then ordering the cover to be replaced. Part of the choreography of glass blowing to preserve heat and ensure that a good product could be blown time and time again, for as long as there was glass in the crucibles.

The results of this work will be published in a short article in London Archaeologist in due course. Our earlier work, at Dock Street, can be found in an article by Richard Humphrey and John Shepherd in the Autumn 2013 edition of London Archaeologist (volume 13, no.10).

by John Shepherd