Sodium chloride is an accelerator up to about 2% of the hemihydrates, but at a higher concentration, it acts as a retarder. Sodium sulphate has its maximum acceleration effect at approximately 3.4%; at greater concentrations, it becomes a retarder.
 
The most commonly used accelerator is potassium sulphate. It is particularly effective in concentrations higher than 2% since the reaction product. which seems to be syngenite (K,Ca[SO4].H1O) crystallizes rapidly. Many soluble sulphates act as accelerators, whereas powdered gypsum (calcium sulphate dehydrate) accelerates the setting rate, because the
particles act as nuclei of crystallization.

 Citrates. acetates and borates generally retard the reaction.

One of the main disadvantages of the air-powder polishing system is the large quantity of aerosols created.

The closest speaking space was suggested by McGrane.

Etiology of Tongue thrust
Fletcher has proposed the following factors as being the cause for tongue thrusting. 

Genetic factors : They are specific anatomic or neuromuscular variations in the oro-facial region that can precipitate tongue thrust. e.g. Hypertonic orbicularies oris activity. 

Learned behaviour (habit) : Tongue thrust can be acquired as a habit.

 The following are some of the predisposing factors that can lead to tongue thrusting:
          a. Improper bottle feeding
          b. Prolonged thumb sucking
          c. Prolonged tonsillar and upper respiratory tract infections
          d. Prolonged duration of tenderness of gum or teeth can result in a change in swallowing pattern to avoid pressure on the tender zone.
          
Maturational : Tongue thrust can present as part of a normal childhood behaviour that is gradually modified as the age advances. The infantile swallow changes to a mature swallow once the posterior deciduous teeth start erupting.

Sometimes the maturation is delayed and thus infantile swallow persists for a longer duration of time. 

Mechanical restrictions : The presence of certain conditions such as macroglossia, constricted dental arches and enlarged adenoids predispose to tongue thrust habit. 

Neurological disturbance: Neurological disturbances affecting the oro-facial region such as hyposensitive palate and moderate motor disability can cause tongue thrust habit. 

Psychogenic factors : Tongue thrust can sometimes occur as a result of forced discontinuation of other habits like thumb sucking. It is often seen that children who are forced to leave thumb sucking habit often take up tongue thrusting.

Porosity in heat-cured acrylic dentures can occur due to several factors, but the most significant cause in this context is the direct placement of the packed resin in water at high temperatures (100 degrees Celsius). When the packed resin is subjected to such conditions, it can lead to rapid polymerization and the formation of bubbles or voids within the material, resulting in porosity. This is because the heat causes the monomer to vaporize, and if the vapor cannot escape, it leads to trapped air pockets.