What do the brackets around certain compositions mean?
A. The brackets, e.g. (0.52), mean that this is an uncertified composition and is given for reference only. A reference value is established with at least one analytical technique, however, analysis using a second or third independent verification would not have been performed. In the current Alcoa catalog, certified values are provided for most compositions. The exceptions to this are the Alcoa “SQ” standards which are used to compensate for instrument drift in the intensity domain.