The Case of the Definite Definitions

Question:

Hi Glass Detective,

Could you please clarify this for me—are the terms float glass, flat glass, plate glass and sheet glass used somewhat interchangeably?

Kind regards,

Gagan M.
United Kingdom

Answer:

Dear Gagan,

Thank you for making contact with the Glass Detective with your questions regarding what I will refer to as “glass nomenclature”. By the way, Glass.com has a Glass Dictionary which you may want to take a look at after we have tried to address your questions with this response. Please be aware that not everyone will be in total agreement with my answers, but I feel that for what you want to know, we are going to end up on solid ground. Let’s call this a brief overview so we don’t offend too many of our “expert” friends. One last thing—I am going to answer your questions not in the order in which you presented them but in an order that I think will be clearer and more helpful.  Here we go!

1. Flat Glass is a broad term that covers everything from float glass, sheet glass, and patterned glass (rolled glasses), to plate glass. Also, processed/fabricated glass products such as mirrors, window glass, tempered glass, laminated glass, insulating glass, bullet-resistant glass and anything and everything else that started out as a flat piece of glass pretty much falls into the category of “flat glass”. To those who work in the glass industry, pretty much all glass other than automotive glass is considered flat glass. This may seem a little silly because even glass used in vehicles started out as flat glass, but the common differentiation is that glass falls into two categories—flat and automotive. Glassware, vases and all of that other stuff has its own categories and is not included here.
2. Float Glass is glass that is manufactured on a float line where a molten ribbon of glass is made by “floating” the liquid glass mixture over a bed of molten tin. As it “floats” over the tin and on to a conveyor system of air or ceramic rollers, it slowly cools and forms a long ribbon of (now solid) glass that is cut and packed for shipping. It is called “float glass” because of the process by which it was formed; literally floating the liquid glass over liquid tin and slowly cooling it (referred to as annealing) to become what we commonly call “plate glass.”
3. Plate Glass was not always produced by the “float process”. Prior to the invention of the float process, plate glass was made by grinding large “plates” or ribbons of rough formed glass until it was clear. Therefore, not all plate glass is float glass, but all float glass is plate glass. However, for all practical purposes, I think it is fair to say that “plate glass” and “float glass” are synonymous.
4. Sheet Glass is made using a series of rollers that draws the molten glass vertically up out of the mixing tank (where the glass is still a liquid). The glass is rolled up vertically, and then as it is cooling, turned horizontally, for further cooling and processing. Sheet glass has historically been used for thinner “flat glass” products. The sheet process has limitations regarding thickness and overall size as well as clarity but was less expensive than the old plate glass grinding process so it became more commonly used in situations where a thinner, typically lower cost but lower quality glass could be used. Residential windows, desk tops, picture frames and so forth were typically made with sheet glass. However, the float process, which is very efficient, now dominates the market for certain thin flat glass products that used to be made using the sheet process. Float glass is considered by most to be of better visual quality and easier to transport and fabricate than sheet manufactured glass. The float process also allows for much larger sizes for thinner glass products.

I hope this information/explanation is of some value to you and I again thank you for contacting the Glass.com Glass Detective.