Technical Support

For the last century or so the first stage of treating a decayed tooth has involved ‘drill and fill’ procedures: the decay within a tooth is cleaned out by drilling and the tooth ‘restored’ by filling the clean cavity with a mercury based amalgam or more recently with a non-metallic filler. However after several such treatments the tooth can no longer be repaired by this technique and more radical restorative procedures are necessary… Click here to read the whole article >

All Charles Booth’s alloys have undergone rigorous tarnish testing not only to the relevant British/European/International Standard but also under even more demanding procedures. In addition our quality control system includes the full analysis of each batch of material to ensure conformance to specification. Generally NHS alloys have excellent performance for almost all restorations and patient types. Therefore in any incidence of tarnishing, the restoration should be examined to ensure that it is of good quality and that the film is easily removed by conventional tooth brushing. However occasionally dental restorations do tarnish, irrespective of alloy supplier. There are several established reasons for this:-

1. Dietary - spicy or acidic food intake;
2. Medication - some medicines can generate an aggressive environment;
3. Oral Hygiene - generally the formation of tarnish films will be prevented by good daily oral hygiene. Even once formed tarnish films are removable with normal brushing. However in many instances what appears to be tarnishing is actually staining caused by the habits of the patient, e.g. heavy smoking, which may not be removed by good hygiene;
4. Mouth Trauma - damage to the mouth or gums can generate changes in mouth acidity;
5. Galvanic Action - there are a wide variety of alloys available to technicians/dentists for producing crowns; therefore particularly where a patient has changed dentist once or twice or where alloys have been changed for cost purposes or NHS considerations, it is possible that a patient might have restorations of completely different compositions in addition to some silver/mercury fillings. In such circumstances tarnish and/or corrosion of the lesser noble alloy(s) is a real possibility;
6. Defects in casting - shrinkage/gas porosity and entrapped oxide aggravate any potential for tarnish, particularly in the circumstances noted above. It is essential to follow the alloy supplier’s recommendations for melting and casting to eliminate these defects. Overheating of the alloy during melting, particularly if using either an oxygen/gas torch or induction heating, where very high temperatures are generated, will cause both shrinkage porosity and oxide formation. The temperature of the ring/mould also has to be controlled to ensure that sub-surface shrinkage porosity is eliminated.

Dentists should be able to identify the potential for tarnish in their patients by considering oral hygiene, medication, dietary, trauma and the appearance of any existing restorations. The dentist should recommend the use of a more noble alloy for the restoration, e.g. Bodent 75 - a 75% Gold + PGM alloy - where there appears to be potential for tarnish with a particular patient. In extreme circumstances Bocraft Y - with a total precious metal content of 97.5% - although a porcelain bonding alloy, can be used for unbonded restorations giving excellent resistance to tarnish even in the most hostile oral environment.

The Bocraft D is suitable for porcelain fused restorations but the porcelain must have a matched thermal expansion coefficient.

CB2 would be the preferred alloy to construct the coronal coping for a telescopic crown. (more accuracy in the casting and easier to mill)

Whether the ceramic can be pressed onto Cobral G depends on what ceramic is being pressed. There are not many ceramics specifically sold for press-on-metal. If the ceramic used is IPS InLine PoM (Ivoclar Vivadent) or similar which has a TEC range of 13.8-14.5 then Cobral G should be fine. It is similar in composition to the IPSd.SIGN 30 alloy supplied and recommended by Ivoclar. I wouldn’t recommend a heavy ceramic shoulder with this combination of ceramic and alloy though.

Cobral G should perform well as a telescopic crown material however, it could be time consuming to finish/polish the casting when thin margins are required.

Traditionally a ceramic with a high Thermal Expansion Coefficient should be used with base metal alloys as the alloy shrinks more on cooling. However the TEC of Cobral G is not particularly high for a Co-Cr alloy. I would always recommend that the laboratory checks the TEC range of the ceramics they have available and match that with Cobral G (14.3).

Laser etching should be fine, although as there is little literature on this for the Co-Cr alloys I would recommend using in combination with grit blasting

We would recommend that the alloy is sandblasted after casting to improve the intimacy of the contact of the porcelain with the metal.

The liquidous temperature is 1190

Metallic taste is a reported complaint of many non-precious alloys. The incidence is not predictable, however it has been suggested that it occurs more commonly when the crown is in contact (galvanically coupled) with another metal. This could be an amalgam filling in the core restoration under the crown or it could be an amalgam or a different metal crown in the adjacent teeth.

We wouldn’t recommend using Bocraft R for the coping portion of a telescopic crown – a hard alloy would be more appropriate. The same applies for bridge work –Bocraft R has a 73.5% high gold content – a palladium based alloy with a much lower gold content would be more appropriate e.g. Bocraft D. It would cover 2 teeth.

The Noritake Super Porcelain EX-3 thermal expansion coefficient (TEC) is 12.4 and is recommended to be used with alloys with TECs ranging from 13.4 to 14.4. Bocraft D has a TEC of 13.9 so sits nicely in this range.