GB1562712A

GB1562712A - Method of making coins

Info

Publication number: GB1562712A
Application number: GB2926676A
Authority: GB
Original assignee: Metallwerk Plansee AG; Metallwerk Plansee GmbH
Current assignee: Metallwerk Plansee AG; Metallwerk Plansee GmbH
Priority date: 1975-07-28
Filing date: 1976-07-14
Publication date: 1980-03-12
Also published as: ATA584175A; AT342875B; CH623731A5; DE2633323A1; CA1086992A

Abstract

A powder metallurgical method, in which the alloy components in the coin blank are distributed inhomogeneously or a foreign additive is distributed in the blank homogeneously or in a localised manner or the metal structure is influenced. By means of these material properties, an additional protection against forgery is achieved. In addition to tungsten, other metals can also be used as foreign additives, such as molybdenum, niobium, tantalum, chromium, zirconium and titanium, but also metal oxides, such as Al2O3, TlO2, ThO2 and ZrO2, provided these are sufficiently resistant to corrosion and do not alloy with the coin metals. Refractory oxides, such as Al2O3, ThO2, ZrO2 and TiO2 are also suitable.

Description

(54) A METHOD FOR MAKING COINS (71) We, METALLWERK PLANSEE AKTIENGESELLSCWT, an Austrian joint stock company, of A-6600 Reutte, Tirol, Austria, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:: The present invention relates to a method for the production of metallic currency coins, or collector coins as distinguished from electrical contacts with the use of coil blanks produced by powder metallurgy from a metal powder to nominal dimensions and nominal weight It has been the general practice to start the production of coins from metal melt and to produce sheets or nbbons of the thickness of the coins to be minted therefrom, to stamp coin blanks from the metal sheets or ribbons, subsequently to apply a finish to the surface of the blanks and finally to mint the blanks. The rather substantial metal waste left over from the stamping has to be reworked and to be added to the new metal charge.

Hitherto the properties of the material of coins have been used for the characterisation of genuine coins to a small extent only; as a rule only the composition of the alloy was checked in a generally known way by measuring the density (specific gravity) thereof.

A purposeful characterisation of the materials of the coins has not been applied, and the characterisation attainable exclusively by powder-metallurgical methods was not obvious even to a small circle of specialists.

By "characterisation" of a coin it is meant giving the coin such distinguished physical properties that the genuine is readily distin guishable from the counterfeit by non-destructive testing.

It would be practically of no use to a potential counterfeiter to know the features of attaining protection from counterfeiting.

The expenses involved in powder metallurgy and the specialist knowledge required therefor would be too great for any unauthorised production of coins to remain undetected On the other hand, it has been known to influence the properties of articles such as electrical components by the addition of sub- stantial proportions of materials such as oxides or other metals to the base material powder for powder metallurgy production in order to increase e.g the strength and/or electrical conductivity of the components, depending on the requirements of their use. The additions have been in general capable of reacting with or alloying with the base metal.

The method according to the present invention of making currency coins, or collector pieces aimed at defeating coin counterfefters comprises producing the coins by powdermetallurgical techniques in which the coins are characterised (as hereinbefore defined) by one or more of the following: (i) the controlled non-homogeneous mixture of powder constituents within the powder compact; (ii) the inclusion within the powder compact of 1% or less of homogeneously or locally distribured material different from the basic materials of the coin which is not reactive or alloyable with the basic metallic material and does not appreciably affect the mechanical or chemical properties; or (iii) the control of the grain 'size' of the powders used in the compact so as to secure controlled grain 'size' in the sintered product.

The invention will now be described in more detail with reference to some typical embodiments:-- EXAMPLE A melted down Ag-Cu alloy may be worked into a powder of an average grain size of about 20 mu. It is convenient to granulate the powder by any known methods For the protection from counterfeiting one admixes less than 1% by weight of tungsten powder of substantially the same grain size homogeneously with the alloy powder. Thereafter an accurately weighed amount of powder is pre-pressed in a double acting press with polished punches to about 60 % of the theoretical density.The green compact is subsequently sintered at temperatures between 650 and 700 C, i.e. clearly below the melting point of the Ag-Cu alloy, for about two hours in a H protective atmosphere. In this step the coin blank shrinks and reaches a den sitv of about 99 / . The tungsten admixture remains homogeneously distributed in the coin blank even after the sintering. This result is attainable exclusively by means of powdermetallurgical methods. If the tungsten addition had been admixed to an Ag-Cu melt, it would have sunk to the bottom of the crucible and thus would have been lost to the subsequent metal sheet production.The dimensions of the blank had been chosen for the green pre-pressed blank so that the blank after sintering is slightly thicker than the nominal dimension, and its diameter is about 3 ,, smaller than the requested nominal diameter. The coin blanks therefore have to be sized after the sintering to their nominal dimensions by a subsequent second pressing operation.

The coin blanks are then blanched (silverwhitened) by a conventional method, i.e. the copper content of the alloy is oxidised superficially and dissolved whereby the silver is enriched on the surface. It is advisable to polish the coin blanks superficially prior to minting. Alternatively, the polishing may be carried out simultaneously with the second pressing, subsequently to the blanching. Testing the genuineness of the coins, i.e. the proof of the foreign tungsten metal and of the homogeneous distribution in the coin, is carried out in a non-destructive way by means of Xrays and evaluation of the intensity of the transmission of the radiation.

When using powder metallurgical methods in the production of coin blanks a great number of other possibilities are available, apart from the protective measures against counterfeiting described hereinabove. By pressingin thin ribbons or strips of a different metal into the green blank, the characterising effect described hereinabove could be likewise attained. As additional different materials, apart from tungsten, other metals could be used such as molybdenum, niobium, tantalum, chromium and titanium, and also metal oxides such as Awl203, TiO2, TbO2 and ZrO2, provided the same are sufficiently corrosion-resistant and do not alloy with the coin metals.

With the aid of the neutron activation method for example the presence in the blanks of minute additions of the oxides of the rare earths such a Gd-oxide and Dyoxide could be checked by their characteristic gamma-radiation.

Ag-Cu coins may. for protection from counterteiting, be composed for example of an outer annular zone with a higher Ag-content, and a core-sone of correspondingly lower Ag-content, or vice versa, provided that for the coin as a whole the required alloy contents remain unaltered. Such shaped pieces having zones of different composition of materials have recently been made. The proof of genuineness could be carried out simply and non-destructively by measuring the electrical conductivity, particularly by eddy-current measurements.

The exact control of the metallic structure likewise opens up a wide range of possibilities within the scope of powder-metallurgical coin production.

The mean grain diameter of the sintered resin coin could be ascertained e.g. by electron spectrography. The mean grain size could accordingly likewise serve as a specific charac teristic of the coin.

The improved protection from counter feiting resides, in the fact that powder-metal lurgy production requires very specialised appliances and a far higher degree of specialised knowledge than is required for the production of coins by melting. The pos sibility of execution is accordingly always reserved to a very small circle of persons.

As a preferred basic material for the making of coin blanks according to the present inven tion a Ag-Cu alloy containing 64 oyO by weight of Ag may be used. As precious metals a platinum alloy or an Au-Cu alloy may be used.

WHAT WE CLAIM IS: 1. A method of making metallic currency coins or collector coins aimed at defeating coin counterfeiters which comprises producing the coins by powder metallurgy techniques in which the coins are characterised (as hereinbefore defined) by one or more of the following: (i) the controlled non-homogeneous mixture of powder constituents within the powder compact; (ii) the inclusion within the powder com pact of 1 1 t or less of a homogeneously or locally distributed material different from the basic metallic materials of the coin which is not reactive or alloyable with the basic metallic materials and does not appreciably affect the mechanical or chemical properties.

(iii) the control of the grain size of the powders used in the compact so as to secure controlled grain size in the sintered product.

2. A method according to claim 1, wherein the rare earth metal oxides Gadolinium and or Dysprosium are used as the different material.

3. A method according to claim 1, wherein the basic powder metal alloy mixture is richer in one alloy component in an outer annular zone of the blank and richer in another component in an inner core zone, the average of said components over the blank as a wholly rorresponding to the nominal ratio of said components in the mixture.

4. A method according to claim 1, where

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. subsequently sintered at temperatures between 650 and 700 C, i.e. clearly below the melting point of the Ag-Cu alloy, for about two hours in a H protective atmosphere. In this step the coin blank shrinks and reaches a den sitv of about 99 / . The tungsten admixture remains homogeneously distributed in the coin blank even after the sintering. This result is attainable exclusively by means of powdermetallurgical methods. If the tungsten addition had been admixed to an Ag-Cu melt, it would have sunk to the bottom of the crucible and thus would have been lost to the subsequent metal sheet production.The dimensions of the blank had been chosen for the green pre-pressed blank so that the blank after sintering is slightly thicker than the nominal dimension, and its diameter is about 3 ,, smaller than the requested nominal diameter. The coin blanks therefore have to be sized after the sintering to their nominal dimensions by a subsequent second pressing operation. The coin blanks are then blanched (silverwhitened) by a conventional method, i.e. the copper content of the alloy is oxidised superficially and dissolved whereby the silver is enriched on the surface. It is advisable to polish the coin blanks superficially prior to minting. Alternatively, the polishing may be carried out simultaneously with the second pressing, subsequently to the blanching. Testing the genuineness of the coins, i.e. the proof of the foreign tungsten metal and of the homogeneous distribution in the coin, is carried out in a non-destructive way by means of Xrays and evaluation of the intensity of the transmission of the radiation. When using powder metallurgical methods in the production of coin blanks a great number of other possibilities are available, apart from the protective measures against counterfeiting described hereinabove. By pressingin thin ribbons or strips of a different metal into the green blank, the characterising effect described hereinabove could be likewise attained. As additional different materials, apart from tungsten, other metals could be used such as molybdenum, niobium, tantalum, chromium and titanium, and also metal oxides such as Awl203, TiO2, TbO2 and ZrO2, provided the same are sufficiently corrosion-resistant and do not alloy with the coin metals. With the aid of the neutron activation method for example the presence in the blanks of minute additions of the oxides of the rare earths such a Gd-oxide and Dyoxide could be checked by their characteristic gamma-radiation. Ag-Cu coins may. for protection from counterteiting, be composed for example of an outer annular zone with a higher Ag-content, and a core-sone of correspondingly lower Ag-content, or vice versa, provided that for the coin as a whole the required alloy contents remain unaltered. Such shaped pieces having zones of different composition of materials have recently been made. The proof of genuineness could be carried out simply and non-destructively by measuring the electrical conductivity, particularly by eddy-current measurements. The exact control of the metallic structure likewise opens up a wide range of possibilities within the scope of powder-metallurgical coin production. The mean grain diameter of the sintered resin coin could be ascertained e.g. by electron spectrography. The mean grain size could accordingly likewise serve as a specific charac teristic of the coin. The improved protection from counter feiting resides, in the fact that powder-metal lurgy production requires very specialised appliances and a far higher degree of specialised knowledge than is required for the production of coins by melting. The pos sibility of execution is accordingly always reserved to a very small circle of persons. As a preferred basic material for the making of coin blanks according to the present inven tion a Ag-Cu alloy containing 64 oyO by weight of Ag may be used. As precious metals a platinum alloy or an Au-Cu alloy may be used. WHAT WE CLAIM IS:

1. A method of making metallic currency coins or collector coins aimed at defeating coin counterfeiters which comprises producing the coins by powder metallurgy techniques in which the coins are characterised (as hereinbefore defined) by one or more of the following: (i) the controlled non-homogeneous mixture of powder constituents within the powder compact; (ii) the inclusion within the powder com pact of 1 1 t or less of a homogeneously or locally distributed material different from the basic metallic materials of the coin which is not reactive or alloyable with the basic metallic materials and does not appreciably affect the mechanical or chemical properties.

4. A method according to claim 1, where

in the different material consists of corrosion resistant metals which do not alloy with the metals of the basic powder alloy mixture.

5. A method according to claim 4, wherein said metals are molybdenum, tungsten, niobium, tantalum, titanium, zirconium and/ or chromium.

6. A method according to claim 1, wherein the different material consists of high melting point metal oxides.

7. A method according to claim 6, wherein the high melting point metal oxides are A1,03, ThO,, ZrO2 and/or TiO.

8. A method according to claim 1, wherein said different material is in the form of powder admixed with the basic powder metal alloy mixture.

9. A method according to claim 1, wherein said different material is in the form of thin wires and/or foils embedded in the basic powder metal alloy mixture.

10. A method according to claim 1, wherein the basic powder metal alloy mixture Is a mixture of Ag and Cu powder comprising 64 ó by weight of Ag.

11. a method according to claim 1, wherein the basic powder metal alloy is composed of a precious metal alloy.

12. A method according to claim 11, wherein said precious metal alloy is a platinum alloy.

13. A method according to claim 11, wherein said precious metal alloy is an Au-Cu alloy.

14. A method according to any of the preceding claims substantially as herein described.

15. A coin when made according to a method according to any of the preceding laims.