Physics and Chemistry of Glasses: European Journal of Glass Science and Technology Part B Volume 58 Number 5 October 2017 187 * Corresponding author. Email sfeller@coe.edu DOI: 10.13036/17533562.58.5.004 Phys. Chem. Glasses: Eur. J. Glass Sci. Technol. B, October 2017, 58 (5), 187–194 1. Introduction Previous studies conducted at Coe College have ex- amined the glass transition width (ΔT g ) among alkali borate glasses. Munhollon et al (1) and Starkenburg et al (2) discovered anomalies in the widths of the glass transition of alkali borate glasses at low alkali con- tents. Masao Kodama has also studied these thermal properties; Kodama’s published T g data exhibit an identical, albeit unmentioned, anomaly. (3) The anomaly has been verifed, and the dataset widely expanded to more fully cover the composi- tional range. Onset defnitions were used for T g and T e . (3) ΔT g is defned as T e −T g . This study describes the anomaly and provides an explanation based on Shakhmatkin et al’s thermodynamic model of glass structure. (4) The basis of the Shakhmatkin et al’s ap- proach is that the glass is composed of structural groupings from various associated crystalline stoi- chiometries. Knowledge of borate crystal structures were taken from Wright et al. (5) The concept of the fctive temperature (the temperature at which the glass structure is frozen in) along with a modifed view of constraint theory was also employed in which the chemical groupings used were the superstructural units rather than the short range structures. (6) We use the superstructural groupings rather than the short range trigonal and tetrahedral borons proposed by Mauro et al (7) since there is no diference in the abundances of the short range units between any of the alkali borate series in this range of composition. 2. Experimental procedures A. Sample preparation Glass samples were made as follows: (1) 6 g of rea- gent grade or beter alkali carbonates and boric acid obtained from the Sigma-Aldrich company were used as the starting chemicals. The compounds were measured into a platinum crucible and thoroughly mixed for 5 min. The composition is described by R, the molar ratio of alkali oxide to boron oxide. (2) The sample was then heated for ffteen minutes at 1000°C. At that point, (3) weight loss of the sample was determined and compared to a prediction made from stoichiometry. Samples matched predicted weight losses to within 0·1 g. (4) The sample was then returned to the furnace and heated for another 5 min, again at 1000°C. (5) Finally, the melt was quenched between two steel plates, resulting in clear glass which was subsequently stored in a desiccator. Our previous work in making glasses, which has been published extensively, has shown that the water content typically makes up a few hundredths of a percent of the total mass when glasses are made in this manner. An anomaly in the glass transition width trends of alkali borate glasses at low modifier loadings A. DeCeanne, A. Poter, K. Richter, D. Starkenburg, B. Perez, T. Munhollon, N. Barnes, E. Troendle, C. Flynn, M. Afatigato, S. Feller* Center for the Study of Glass, Coe College, Cedar Rapids, Iowa 52402, USA E. Zanoto & O. Pietl Federal University of São Carlos, São Carlos, São Paulo, Brazil Manuscript received 30 January 2017 Revised version received 22 March 2017 Accepted 24 March 2017 The glass transition onset temperature (T g ), the glass transition end temperature (T e ), and the glass transition width (ΔT g ) of alkali borate glasses were determined using diferential scanning calorimetry (using the defnition ΔT g ≡T e −T g ). An anomaly was found in ΔT g trends of the lighter alkali modifers, with that of lithium being the most prominent. From R=0·03 to 0·10, where R is the molar ratio of alkali oxide to boron oxide, ΔT g values of the lithium family experience a large increase, maximizing at R=0·05. Those of the sodium family experience a smaller increase. After R=0·10, the trends of all alkali borates follow a similar monotonic decline. An explanation based on thermodynamic modelling of the borate species, as well as viscosity, is proposed.