State of the art It is known that during the melting processes of ingots of alloys of precious metals or similar for the moulding of different objects, both for aesthetic or industrial purposes, oxides are formed both on the surface and inside the crystal lattice of the metals constituting the alloy. These oxides can be detrimental to the subsequent use of the objects for example by altering their external appearance or causing more serious problems. For example in the case of parts in precious alloys commonly used in dental application (capsules, bridges, pins, inlays, etc.), when these are implanted in the mouth of a patient the oxides can, under the saliva action, be attacked and thereafter absorbed by the patient, similarly the properties of the parts of precious alloys used in electronics can be adversely affected by the formation of oxides.

It is therefore evident how important it is to make available a product which allows the removal of the oxides from precious metals alloys.

Detailed description of the invention The present invention allows one to overcome the above said problems through the use of compositions of gelatine containing sodium borate.

Such compositions once spread on the objects to be treated and thereafter heated, allow the total removal of superficial and internal oxides possibly formed during the previous working phases of the metal alloy and allow the preparation of products which do not present any of the drawbacks caused by the presence of the oxides.

According to the invention a precious metal or similar alloy is an alloy consisting of metals of the groups VIII and lb of the Periodic Table of Elements, in particular

alloys consisting of gold, silver, palladium, iridium, platinum, in all their different possible combinations.

The gelatine in the composition according to the invention is a gelatinous material capable of burning completely when heated, without leaving any residue.

Homogeneous materials, i. e. lump-free materials, are preferred since they allow a better spreading of the composition.

Particularly preferred for the invention purposes are gelatines such as agar-agar, carrageenin and similar or pure cellulose.

The two products forming the composition, i. e. gelatine and sodium borate, can be present each in a quantity comprised between 20-70% (by weight) of the total; particularly preferred is a composition consisting of 50% sodium borate and 50% gelatine (by weight).

The compositions according to the invention can also obviously be used to remove the oxides in the case of objects wherein the precious metal alloy is combined with resins or ceramics making it possible to obtain products having enhanced brightness.

The preparation of a composition according to the invention is very simple and is performed by mixing the desired quantities of both components in a mixer, preferably equipped with microniser, under vacuum and in a nitrogen current.

Very simple is also the use of the compositions according to the invention.

The piece to be treated, obtained by moulding the melted ingots in the appropriated mould, is cooled, polished and cleaned with vapour jet streams.

The composition according to the invention is spread with a brush on the surface of the object and dried, normally at 500°C; after drying a white chalk-like patina forms.

The alloy is thereafter submitted to thermal annealing or homogenised in oven, normally at 950°C, as indicated by the producer of the alloy.

The object is cleaned by ultrasounds and immersion in pickling liquid or hydrofluoric acid 40%, thereafter neutralised in water with bicarbonate and washed with water.

If alloy-ceramic products are desired the alloy is ceramised according to the usual techniques.

If alloy-resin objects are desired the above said thermal treatment can be performed in an oven or over the flame.

The composition according to the invention is particularly useful to remove the oxides on metal parts which must be grinded.

In the case of products requiring ceramisation, after elimination of the glasure excess the composition is applied to the metallic part and is heated at a temperature largely inferior to the one requested for the ceramic burning, thereafter the same procedure described above is applied (treatment with ultrasounds, pickling, neutralisation and washing).

By operating as described it is possible, for example, to completely remove the oxides in the internal part of the ceramised dental cups making the surface particularly bright.

Example 30 g of carrageenin are placed in a mixer under vacuum and in nitrogen current and 30 g of sodium borate are added. The products are mixed up to the formation of an homogenous paste which is collecte and stoked in fridge up its use.