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Ohinemuri Regional History Journal 14, October 1970

By OWEN J.MORGAN

(The Paeroa Historical Society was privileged this year to have Mr. Morgan, a graduate of the Waihi School of Mines, as one of its guest speakers. His talk on the background of Waihi, which owed its existence to the discovery of gold, led to a discussion on further developments and members were most interested to hear about his own work in connection with the "Dominion Gold Supply Co." This Company has the distinction of now being the oldest secondary industry in the town and at our request Mr. Morgan has recorded its story.) Ed.

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The use of metals for dental purposes dates back at least 300 years B.C., when gold was employed in Rome for fixing artificial teeth. It is however only about 100 years since metals have been systematically used in dentistry, and it was the application of metallurgical knowledge to the production of these specialised materials that brought about dental metallurgy. N.Z. entered this field in 1921, when my father, the late A.H.V. Morgan, M.A., Director of the Waihi School of Mines, began the manufacture of dental gold alloys and in a short time was able to show that the local product was equal in every way to that being imported, and so the foundations were laid for an important and expanding industry which now supplies gold and other tooth-filling materials to dentists throughout New Zealand and overseas.

Gold used in dentistry must be of absolute purity and this is achieved in this Waihi factory by an electrolytic process in which impurities are separated in a strong hot solution of aqua regia and gold chloride. Using this pure 24 carat gold as a base, other metals such as silver, platinum, palladium and copper are added in varying proportions to produce alloys required for a particular purpose by the dentist. Great care is necessary in their preparation, as even the smallest trace of impurities such as lead or antimony renders them brittle and. unworkable.

Through the years other dental materials have been added to the range of products, and a unique claim of the company is that nearly all young New Zealanders are its customers (unenthusiastic though some may be) as its main product is dental copper amalgam, the restorative material prescribed for use by the Department of Health in its school dental clinics. Copper amalgam is made with finely divided copper which is precipitated from its sulphate solution by means of zinc plates. After washing, this flocculent precipitate is amalgamated with pure mercury, dried and while still plastic, rolled into sheets of uniform thickness which are cut into small pellets and left to harden before being weighed and packed. The pellets need only to be heated for them to soften, ready for insertion in the cavity of the tooth, where hardening again takes place.

Copper amalgam has proved very satisfactory for repairing children's first teeth as it can be easily manipulated under conditions which are sometimes difficult. Over one ton of this amalgam is sent from Waihi each year to be used by school dental nurses, and a further quantity is exported to Australia and other countries where school dental clinics modelled on the New Zealand system have been established.

The dental silver amalgam alloy made in Waihi meets a steady demand from dentists all over the country - an increasing quantity being sent to customers in the United Kingdom and Canada. The composition of this most widely used dental filling is silver, tin, copper and zinc which are melted together in an oil-fired furnace. The resultant alloy is then converted into a fine powder by a specially adapted automatic lathe. Further heat treatment anneals the powder before it is graded and weighed into bottles ready for use. After mixing with a weighed quantity of mercury, the dentist packs it into the tooth where it hardens to become the familiar silver filling.

To meet the very rigid specifications laid down by the Commonwealth Bureau of Dental Standards, manufacture of dental materials is precise and exacting. Metals used: silver, zinc from Australia; tin, Malaysia and mercury from the Almeden mine in Spain. Until 1952, when mining operations stopped, all the silver used in these Waihi products was locally produced, for the Martha mine was eight times as rich in silver as in gold, and during its lifetime was of great value to the country's economy, in the same way as the Dominion Gold Supply Co., with its home and export markets is today.