Glass-Transition Temperature (Tg)
Definition
The temperature below which a glass behaves as a rigid solid rather than a viscous liquid; a fundamental thermal property that varies with composition and determines thermal stability in fire or accident conditions.
- Definition
- Temperature marking transition from rigid to viscous glass state
- Property type
- Composition-dependent thermal property
- Fire relevance
- Governs glass deformation under fire heat
- Below vs above Tg
- Rigid solid below; softens and flows above
Common questions
Why does Tg matter in fire scene glass examination?+
Glass exposed to fire temperatures approaching or exceeding its Tg can soften, sag or deform before fracturing, and examiners use these deformation patterns alongside fracture analysis to estimate heat exposure and distinguish fire damage from mechanical breakage.
Is Tg a fixed value for all glass?+
No, Tg varies with the glass composition, soda-lime window glass, borosilicate glass and tempered glass each have different transition temperatures, so the value depends on which glass type is being examined.
How does Tg differ from a melting point?+
Glass is amorphous and has no sharp melting point; Tg marks a gradual softening range rather than a distinct solid-to-liquid transition, unlike crystalline materials which melt at one defined temperature.
Related terms
- ASTM E2927
- Standard test method for the determination of trace elements in glass samples using LA-ICP-MS; specifies the element suite, calibration standards (NIST SRM...
- ENFSI ENG3-2013
- The ENFSI Glass Expert Working Group guideline for glass comparison casework; specifies the examination sequence, within-source variation requirements, statistical comparison framework, and...
- Float Process
- The Pilkington 1959 manufacturing method in which molten glass is poured onto a bath of molten tin, producing a ribbon with two...
- LA-ICP-MS
- Laser ablation inductively coupled plasma mass spectrometry; a technique that fires a laser pulse at a sample surface, vaporises a tiny amount,...
- Network Former
- Oxide (SiO2, B2O3, Al2O3) that forms the tetrahedral backbone of a glass network by sharing bridging oxygen atoms between units.
- Network Modifier
- Metal oxide (Na2O, CaO, K2O, PbO, MgO) that introduces cations into the glass network, breaks Si-O-Si bridging bonds, and lowers the melt...
- PVB Interlayer
- Polyvinyl butyral film bonding the two glass layers in a laminated automotive windshield; retains broken glass in place after fracture, preserving the...
- Refractive Index (RI)
- A physical property of glass measuring how much light bends as it passes through. RI is the primary discriminating characteristic used in...
- Tempered Glass
- Glass heat-treated to introduce compressive surface stresses and tensile core stresses, producing a material four to five times stronger than annealed glass...
- Tin-Side Fluorescence
- UV fluorescence (blue-white at 254 nm) of the bottom surface of float glass caused by tin oxide diffused into the glass surface...
- Within-Source Variation
- The natural variation observed among multiple samples collected from a single source location. Used to estimate the probability of the observed difference...