Topic: Slag

[Frank]

Somewhere there was a discussion about the use of the term slag, but slag gets a lot of hits... so if anyone can remember the topic this could be added to it.

I keep an eye open for the use of the term and today found some references that do not provide any answers but... a bit more on the subject:

1. SILICATE SCIENCE. VOLUME II. GLASSES, ENAMELS, SLAGS. by Eitel, W. 1965


2. Extract from German Glass Industry, 1945 (Again, note it is English Glass technologists reporting)
The following is a brief description of the method of preparing blocks of optical glass approximately 48 x 48 x 14 inches by casting in iron moulds. The batch was mixed in a rotary drum at one end of an L-shaped building. It was then moved in a car (or by an overhead crane) to the pot furnaces. Here it was charged into the pots according to standard practice. The furnace room included ample pot heating furnaces and six single pot melting furnaces were nearly ready to light up. All of these furnaces showed careful design and construction although they were somewhat different from those sometimes used. Both air and gas pressure were carefully controlled. Each furnace had its own air and gas regenerative chambers on both sides of the furnace. The combustion ports which were about 6 in. by 8 in. in size, were cleverly placed somewhat above the furnace floor and directed upwards along a “bee-hive” curved side-wall and cap which started at the floor on one side and arched clean over to the ports and floor on the other side. This cap which was of silica brick about 6 inches thick, was insulated with about 12 inches of natural silica insulating brick. All of these cap blocks were fitted carefully and then sealed with a thin coating of high temperature cement before application of the insulation. The front and back walls were straight with the curved arch between them. Pot stools rose about 8 inches above the floor.
A 6 inch slag hole was located at the centre at the rear and all the floor sloped toward this. The door was in one piece covering the whole end of the furnace and it had a stirring and observation hole about 6 inches high and 18 inches wide, located just above the pot brim level. It was about 12 inches thick and was removed by a heavy screw winch device. It was of ordinary good quality coarse grog and fireclay construction and had a projection which fitted into the end of the furnace when the door was closed. The stirring machines appeared to be of very substantial design but very similar to older standard types. The pots were about 38 inches deep and 48 inches in diameter.



3. Advert for THE P.B. SILLIMANITE CO. LTD - Pottery Gazette and Glass Trade Review, January 1950

Furnace Construction
All parts exposed to products of combustion, such as combustion chambers, flues, doors, pillars to support heavy loads at high temperatures, recuperator tubes, etc., etc. All parts exposed to temperature fluctuations and shock. Resistant linings for withstanding the action of many slags, moving charges, abrasion and any type of fuel. Pyrometer tubes.

4. Soda-slag as a raw material for the glass and enamel industry. Sprechsaal 1943, 76, No. 17 (Note while in English this article was a German research paper, so it might be a translation to soda-slag. Does anyone have any copies of Sprechsaal?

extracts courtesy the Glass-Study

[Ivo]

slag as used in "slag glass" is specifically blast furnace slag, a residue product from the smelting of iron ore. Slag of various sorts  is produced where smelting, sintering and refining take place, including in the smelting of glass itself. So if a paper refers to a slag hole, this would be the repository for the residue of the melt which floats to the top - the scum - which is raked off. Quite a critical operation in optical glass, it seems. Even more so than in soup making.

[Frank]

 

[Adam]

All multi-pot furnaces must have provision for getting rid of the glass escaping within the furnace from cracked, or holed pots.  This might be slow dribbles or up to a ton suddenly sloshing around from a catastrophic breakage.  These provisions take the form of dedicated channels leading to what we knew as "pockets" - simply chambers in the base of the furnace with iron doors.  When necessary these would be emptied, usually with the help of portable burners.  A lousy job.  I can't remember if we had a (printable) name for the mixture of glass, bits of pot and sometimes bits of furnace which came out.  It certainly wasn't "slag" in our case but this seems an appropriate name for someone to use and that is what will be meant in this case.

Adam D.

[Frank]

I have found a great 19th century account of slag glass, as described by Ivo, which also explains why it is hardly known outside Europe. Will post an extract her when I get a chance... it is lengthy.

[Sklounion]

Great, Frank,
It will help to clarify what these terms mean.
Like many of us, terms do not transpose/translate exactly in translation, leaving us not confident, that the translation actually corresponds with what we think the term means......... plus terminology can change from one century to the next....... so we are always chasing the game.
Marcus

[Frank]

Here is some small extracts of information on 'slag' from the Glass Study, 19th century source, as promised above and subsequently forgotten... 

In the manufacture of iron with blast furnaces there is formed more or less of a dark-colored glass known as slag or scoria.

Within three or four years the manufacture of articles made from slag has been carried on successfully in England in the iron districts. In the Cleveland district alone from three to four million of tons of slag are made annually...

... In England we read of an iron manufacturer offering to furnish to any glass-maker who wishes to establish glass works in his neighborhood three million of tons of slag for nothing...

...Slag-glass, in its natural state, is too dark to be applied to the purposes for which ordinary white glass is used, but it can readily be made useful in all articles of cheap and ordinary use where slightly colored glass is not objectionable. As a competitor against ordinary glass, slag-glass has a great advantage in its low price, since the larger part of its ingredients could be had for nothing....

Since last adding to this thread I came across a lot more info, some

There is a lot more and its use was also known in the UK in the 17th century. Also in Bohemia, probably well before then.

[Frank]

Slag glass was also used to make glass fibre insulation in the US at Clove Furnace, NY, by an intriguing process. In Austria it was also used for making glass fibre but woven into clothing accessories and lace. (Mid 19th century).