All Dental Cements used directly in the mouth can be divided into three classes.

1. Zinc Oxid Cements the powder of which is primarily zinc oxid.

2. Copper Cements where the greater part of the powder is a copper compound generally an oxid.

3. Silicious Cements or Plastic Porcelains where the powder is a basic aluminum silicate of some form.

The liquids are all ordinary phosphoric acid containing dissolved phosphates.

The zinc oxid cement is the most universally used of all dental cements and can be considered as a type. It is comparable in a way to ordinary Portland Cement. In a Portland Cement we have a material capable of taking up water, hydrating, crystallizing and cementing together various quantities of crushed stone or sand. In a zinc oxid cement we have the formation of a zinc phosphate of some kind which crystallizes and cements together a certain amount of the original powder not dissolved. There is this difference however with a Portland Cement, the question of time of set, shrinkage, color and other properties which

are of paramount importance to a dental cement, need not be considered.

Every dental cement is made by mixing together a powder and a liquid. The powder of zinc oxid cement is generally composed of 90 to 100% of zinc oxid and the liquid is ordinary phosphoric acid. Now if you mix ordinary zinc oxid and phosphoric acid together a vigorous chemical reaction takes place and progresses so rapidly that a crumbly mass is obtained of no cementing value. It is necessary to slow down the reaction just as potassium bromide is added developer in photography to make the development of the negative take place more slowly.

to

a

It is possible to retard the setting qualities of the zinc oxid and the phosphoric acid and both methods are used. With the zinc oxid there are three methods of

retardation.

1. The greater density a substance has the more slowly it reacts chemically. The obtaining of the proper density of the zinc oxid determines largely the value of the cement. Take a bar of steel, cut it in half and temper one half. Both pieces have the same chemical composition but differ markedly in their physical

properties. The same is true of various forms of zinc oxid.

2. Foreign substances can be incorporated with the zinc oxid to retard the setting qualities. Bismuth and Magnesium Oxids are the foreign substances generally used.

3. Chemical activity is proportional to the surface exposed, hence the powder must be ground to the proper fineness. A coarsely ground powder gives a slower setting cement but the cement is not suitable for inlay work. A finely ground powder gives a faster setting cement. The additional care required in mixing is compensated by better results. The liquid is retarded by dissolving aluminum phosphate in the phosphoric acid and by obtaining the proper specific gravity of the liquid.

What takes place when the cement is mixed? If zinc oxid is added to the phosphoric acid liquid in making a mix of zinc oxid cement, there is first formed an acid zinc phosphate. Chemical activity is greatest in the early stages of the mix, hence care is necessary to bring the powder into the liquid in small portions. in the early stages of the mix, with thorough spatulation, otherwise small hard particles will be formed. As more powder is added to the liquid, more acid is neutralized and the reaction is much slower. The powder can then be added in larger quantities. It is just as necessary to spatulate a dental cement mix as it is to build a machine to thoroughly mix a Portland Cement. Chemical activity is proportional to the temperature and a cool slab is essential to retard the setting qualities. Prolonged spatu

lation retards the setting qualities and fast spatulation accelerates it. The resultant mass contains zinc phosphate cementing together undissolved particles of the original powder. The water of the liquid is taken up as water of crystallization.

Dental Cements are made to give the best results at 70° F. or slightly below that temperature. A prompt setting cement is demanded by the Dentist. It is impossible to make a prompt setting cement which is not sensitive to temperature changes. The liquid is chiefly phosphoric acid which is hygroscopic and changes rapidly on exposure to the air.

The atmosphere can be blamed for a large percentage of the troubles which dental cements encounter. The atmosphere contains moisture in varying quantities depending on climatic conditions. A hygroscopic body or liquid is one which absorbs moisture when exposed to the air. A substance effloresces when it gives up moisture to the air. Then there is the surface absorption of moisture by bodies which does not come under either of the above classes. These conditions can effect a dental cement in three ways, the powder, the liquid and the mixed cement. These changes are familiar to everyone.

The dental cement powders are not affected by atmospheric conditions to any great extent. There is some surface absorption of moisture but it is very small, rarely over 1 or 2%, generally less and it is practically negligible as far as influencing the mix. The absorption of carbon dioxide. from the air need not be considered with any dental cement