What is the gold standard material historically used in clinical trials/research studies as the reference standard for studying dentinal bridge formation?

For decades, calcium hydroxide has served as the established reference/comparator ('gold standard') material against which newer pulp-capping materials (like MTA and bioceramics) are tested in research on dentinal bridge formation, even though many newer studies now show MTA/bioceramics outperforming it in bridge quality and predictability (see the next question).