kitchenware, sculpture, as well as for artistic glass smoking pipes. Lighting manufacturers use borosilicate glass in some of their lenses. Organic light-emitting diodes (OLED) (for display and lighting purposes) also use borosilicate glass (BK7). The thicknesses of the BK7 glass substrates are usually less than 1 millimeter for OLED fabrication. Due to its optical and mechanical characteristics in relation with cost, BK7 is a common substrate in OLEDs. However, depending on the application, soda-lime glass substrates of similar thicknesses are also used in OLED fabrication. Optics Many astronomical reflecting telescopes use glass mirror components made of borosilicate glass because of its low coefficient of thermal expansion. This makes very precise optical surfaces possible that change very little with temperature, and matched glass mirror components that "track" across temperature changes and retain the optical system's characteristics. The Hale Telescope's 200 inch mirror is made of borosilicate glass. The optical glass most often used for making instrument lenses is Schott BK-7 (or the equivalent from other makers, such as the Chinese crown glass K9), a very finely made borosilicate crown glass. It is also designated as 517642 glass after its 1.517 refractive index and 64.2 Abbe number. Other less costly borosilicate glasses, such as Schott B270 or the equivalent, are used to make "crown-glass" eyeglass lenses. Ordinary lower-cost borosilicate glass, like that used to make kitchenware and even reflecting telescope mirrors, cannot be used for high-quality lenses because of the striations and inclusions common to lower grades of this type of glass. The maximal working temperature is . While it transitions to a liquid starting at (just before it turns red-hot), it is not workable until it reaches over . That means that in order to industrially produce this glass, oxygen/fuel torches must be used. Glassblowers borrowed technology and techniques from welders. Rapid prototyping Borosilicate glass has become the material of choice for fused deposition modeling (FDM), or fused filament fabrication (FFF), build plates. Its low coefficient of expansion makes borosilicate glass, when used in combination with resistance-heating plates and pads, an ideal material for the heated build platform onto which plastic materials are extruded one layer at a time. The initial layer of build must be placed onto a substantially flat, heated surface to minimize shrinkage of some build materials (ABS, polycarbonate, polyamide, etc.) due to cooling after deposition. Depending on the material used, the build plate will cycle from room