Toughening of glass is done after cutting to size and any etching or edging processes. The glass is heated to over 600 degrees centigrade then quenched, cooled by air jets. In the photo the mouth of the furnace is on the left, the hot glass has travelled on rollers and is on the right.
For photo of testing see TQ1982 : Preedy Glass, impact testing toughened glass For an outside view of the factory and a description see TQ1982 : Preedy Glass, Coronation Road, at Open House Festival.
The Guinness Brewery photo appears to display Moiré patterns. Is this a problem with Geograph, or a problem with the original photograph? I suspect Geograph, as many images appear to display quite jagged lines.