Prosthetic dentistry / by James Harrison Prothero.

  • Prothero, James Harrison.
Date:
1916
Catalogue details

Licence: Public Domain Mark

Credit: Prosthetic dentistry / by James Harrison Prothero. Source: Wellcome Collection.

    77/1222 (page 51)
    The foregoing extracts refer especially to the chemistry of setting, and the effects of varying temperatures and time in burning on the time of. setting and hardness of plaster. The method of manipulation is not considered. This to the prosthetist is a matter of importance that cannot be over- looked. INFLUENCE OF MIXING ON QUALITY Plaster mixed at a medium to thick consistency will set more rapidly and be of better density when set than if an excess of water is present. Too thick a mix, however, should be avoided, particularly where accelerators are used, as under such circumstances the setting frequently begins before the mass is of uniform consistency, and further stirring breaks up the already forming erystals. This interference results in a mass of uncertain density, oftentimes with many spaces present. The other extreme, too much water, should be avoided, as it retards the setting, and the plaster, when set, is less resist- ant to stress. Various substances are added to accelerate the setting, particularly in impression work. Common table in the presence of steam or water. Sulphate of potassium (IxX,SO,) is by far the best accelerator, because it not only hastens the setting, but has a controlling influence on expan- sion. This medium, however, should not be used in cast con- struction for the same reasons that apply to NaCl. Potas- sium alum has been highly recommended as an accelerator and hardening medium for plaster in dental laboratory pro- cedures, but tests made with this substance fail to verify the claims made for it. EXPANSION OF PLASTER ~ — ting and the deleterious effects of this movement in denture adaptation have long been recognized by some, but its impor- tance is not fully understood. When freshly mixed plaster begins to set, it contracts slightly, then remains stationary for a short time, then ex- pands, and after assuming a thoroughly dry condition it again. in temperature is noticeable during the setting process, due to the chemical action that occurs. |