In a recent project I have been restoring windows dating back to 1704. This colonial farm house has been well preserved and the original structure has most of its original glass. It offered a great opportunity to examine and document historic glass from this period in the Mid-Atlantic region of the United States.
The majority of the glass is true "single strength" mouth blown antique glass. While we rarely use single strength or less than 1/8" glass in our windows today this historic glass has an average thickness of .055" - .08", (1.4 - 1.8 mm), which is nominally 1/16" or slightly less. By any modern standards this if very thin glass and challenging to work on.
Modern residential window glass which is typically called "double strength", (1/8" with a decimal equivalent of .125", or 3.17mm referred to generally as 3mm glass, is standard and thicker glass is often used in larger applications, for insulated glass units, security or sound dampening situations.
Once I got all the glass out of the sash and carefully cleaned it there were an astonishing number of variations in each piece. It has a totally unique charm and authenticity that while challenging to work on was very satisfying. (as I stated in a previous post, I am a glass geek) One of the more interesting characteristics along with the "seeds" or air bubbles, striations or "straw marks" there were also a wide variety of "inclusions" and reamy marks.
So what are "inclusions"? Inclusions in glass can be any number of foreign material that have become trapped in the glass. They vary from unmelted "batch" or the raw materials of soda-lime or quartz sand that stays in its solid state and did not properly melt into liquid glass, (typically white in appearance) and other imperfections that can be metal oxides and debris trapped beneath the surface of the glass, (more often appearing as dark brown or black specs).
Working to recapture these characteristics in RenovoGlass™ is an ongoing process to make a truly authentic restoration glass with the nuance of hand blown glass and bringing it up to date to meet modern building requirements. We will be updating our process to include RenovoGlass™ Plus to incorporate "inclusions" in our upcoming product line. I'll talk more about RenovoGlass™ and the different "schedules" of glass that distinguish the various levels of distortion and "imperfections" in upcoming articles.