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THE MAKING OF A CROWN

 

This starts after the dentist has done his work of preparing the tooth for the crown, when the technician who is to make the crown comes into the treatment room to assess what colours and densities of porcelain he will need to make the crown to match exactly to your natural teeth.This can involve up to four stages of assessment, using different shade guides to achieve the final result.

Next, the impressions taken by the dentist in the preparation of the crown are cast by the technician to make accurate models of your mouth. This stage uses the skills of selection of the right type of plaster, its correct mixing and accurate pouring, to produce a model dimensionally accurate to within thousandths of a millimetre, and clean of any air bubbles. This model has to be finished, and trimmed, and dies (accurate individual models of the prepared teeth) produced of the teeth that are to be crowned.

The chassis

imageThe inner layer of a crown is often made of cast metal, and the casting is produced by a process as old as metalworking itself – a method used by the ancient Egyptians, and still used today by gold- and silversmiths to make jewellery – the lost wax method. The technician uses a special high quality waxes to form a shell, one third of a millimetre thick, over the die. This wax shell, or pattern, is invested under vacuum in a metal casting ring, using special plaster which is designed to shrink on setting by the same amount as it will expand when it is heated to casting temperature, around 1,000 degrees Centigrade. This ensures that the casting will end up the same size as the wax pattern. After setting, the plaster is heated in a furnace and the wax evaporates out, leaving a clean mould inside it. The plaster is then placed into the casting machine which heats the casting metal until it is a free-flowing liquid, and then spins it so that it runs into the plaster mould by centrifugal force.

Making the crown look like a tooth

imageWhen the metal shell has been cast and cooled, it is separated from its plaster mould and cleaned by sand-blasting. The surface is prepared to make the porcelain layers of the restoration stick to it and porcelain is added by the technician. This is one of the most skilled procedures. The porcelain comes as a powder of coloured glass, which is mixed to a slurry with distilled water on a palette, and applied to the metal shell with an artist’s paintbrush. The first layers are dense and opaque to hide the dark metal inside, and outer layers are progressively more translucent, just like the make-up of a natural tooth. This mimics its colours and translucencies. As the layers are added details and characterisations are added by the technician to mimic the teeth around the final crown when it is in place in the mouth. The skill of this lies in two things. The first is that the layers of slurry look nothing like the finished result until the crown had been fired by passing it through a furnace, under vacuum, to allow the particles of glass to melt and flow together. The second is that when the porcelain mass fuses in the furnace the volume of the mass reduces as the space between the glass particles disappears, so that the technician has to build the mass over size, for it to shrink back to the correct size in the furnace.

What goes in as an opaque, matt white blob, comes out tooth coloured and translucent. The technician cuts, trims, and sculpts the fused porcelain to shape it more exactly to match your other teeth, and when it is finally right it is coated with glaze and passed through the furnace again, to produce the degree of gloss and surface texture that the dentist and the technician have agreed upon.

Finally it is cleaned and polished to produce the jewel-like result that we bring to your mouth to restore function appearance and comfort to you.

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