26. Measurement of initial and final setting time of plaster materials
Explains the two setting moments of plaster, initial and final, and how each is measured with the Vicat and Gilmore needle devices and with an empirical glass-slab method. It covers the practical signs of setting, then a table of factors that speed or slow setting, including temperature, water ratio, mixing and accelerators or retarders.
Topic 26 of 52 in the MU Sofia syllabus for Dental Materials (Dental Medicine, Year 1). The free AceMed note covers it in 6 sections, about 271 words (3 min of reading), with 13 flashcards.
In this note
Introduction
Key terms
Instrumental methods
Empirical method
Signs in daily practice
What changes the setting time
Introduction
Setting time tells the technician how long there is to pour a model and when it can safely be handled. Two moments are measured: the initial and the final setting time.
Key terms
Working time: from the end of mixing to the initial set. The mix flows and can fill the impression.
Initial setting time: viscosity rises sharply as dihydrate crystals appear, so pouring must stop.
Final setting time: the crystal network has formed, and the model can be moved and shaped.
Instrumental methods
Needle devices use a stand, a rubber ring 5 cm tall filled with the mix, and a weighted steel needle that slides in a holder. A ruler shows how far the tip travels.
In the Vicat method, the needle is released every 15 seconds from the start of mixing, each time at a new spot. The initial set is recorded when the tip no longer reaches the bottom of the ring. The final set is recorded when the tip sinks only 1 mm. The Gilmore method works in a similar way.
Continue reading free
The whole note, all 6 sections, is free with an AceMed account, with its 13 flashcards. Premium adds flashcard recall, highlights and Ask AI.
