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Page 1 Heraeus Ltd., Dr. P. H. Ko, FM, Surabaya, Apr 2011 Understand Better about Jewelry Production Dr. Po-Hung KO BU/Global Manager Functional Materials Understand Better about Jewelry Production

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Page 1: Seminar Heraeus

Page 1Heraeus Ltd., Dr. P. H. Ko, FM, Surabaya, Apr 2011

Understand Better about Jewelry Production

Dr. Po-Hung KO

BU/Global

Manager

Functional

Materials

Understand Better about Jewelry Production

Page 2: Seminar Heraeus

Metals for Jewelry

Productions

Page 3: Seminar Heraeus

Page 3

Metals for Jewelry Production

Pure gold gives the unique yellow color but it is too soft (22HV) for making jewelry.

Addition of other metal(s), usually in the form of master alloys can increase the strength of the gold alloys.

The addition of other metals can also change the colors of the resultant karat gold. We can therefore have more color varieties in the karat gold production.

Page 4: Seminar Heraeus

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Colors of Karat Gold Alloys

18K Ag

Zn Cu

Pink

Light

Pink

Pinkish

yellow

Green

Yellow

Greenish

yellow

Greyish

brown

Light

brown Light

yellow

Light

green

Yellowish

green

Page 5: Seminar Heraeus

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How the Colors Change in Gold Alloys?

Silver gives the greenish color for the gold.

Copper enhances the reddish color of the gold.

Zn can bleach the gold color and reduce the reddih tint of the gold color.

The presence of nickel or palladium can whiten the gold alloy.

Fancy color such as purple gold can also be made with the addition of aluminum. The color is due to the formation of intermetallic compound.

Page 6: Seminar Heraeus

Basic Metallurgy

Page 7: Seminar Heraeus

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Nature of Metals & Alloys

Malleability is a unique character of metals.

Pure metals are usually soft. Alloying the metals can increase the

mechanical properties of the alloys. Besides, it can change the color

of the metals.

Not all metals can be

miscible. It depends on

metals‘ crystal structures.

Totally miscible: Au/Ag

Partially miscible: Ag/Cu

Immiscible: Ag/Ni

Page 8: Seminar Heraeus

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Phase Diagrams

Phase diagram is like a road map for the behaviours of the alloys in

different temperature and metal ratio.

It gives very useful information to tell the behaviour of the alloys.

Examples of Phase Diagrams

Immiscible Miscible Partially miscible

Page 9: Seminar Heraeus

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Phases in AlloysA phase is a substance that is chemically & structurally

homogeneous withi itself but is physically separated by definite

boundary surfaces from other substances.

If the metals are soluble to each other, they will form solution which

is regarded as single phase.

Too many phases in alloy will increase the hardness and reduce the

ductility.

Page 10: Seminar Heraeus

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Solidification of Metals

When the metals started to solidify, they will first form a nucleus. The

crystal started to grow with crystalline structure.

Grain refiner or just insoluble impurities can act as nuclei to

stimulate the crystallization.

The crystallites grow uniformly in all

three directions until they meet the

adjacent growing grains.

The composition of crystallites may

change during solidification known as

coring.

Fast cooling can suppress the coring

and homogenize the composition.

Page 11: Seminar Heraeus

Why Master Alloys ?

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Why Master Alloys?Master alloy is a concentrate of various metals and additives to offer

the resultant alloy special properties. Many industries have their

particular master alloys.

Jewelry industry has been using silver, brass and nickel to make the

alloys for decades.

Modern master alloys have been developed for various applications

according to the ways how they are handled.

Some alloys are designed for hand-pouring and some are for

machine-casting and machining purpose. Different additives are

added in these master alloys accordingly.

Careful selection is a must. Otherwise, undesirable defects will

appear and consequently lower the productivity.

Different karatages may sometimes need different alloys.

Page 13: Seminar Heraeus

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Common Ingredients in Master Alloys

Additives Functions

Silicon (Si) • Deoxidizer to remove the oxygen & increase fluidity

• Cause cracking problem

Germanium

(Ge)

• Deoxidizer & increase tarnishing resistance

• Porosity problem & expensive

Indium (In) • Increase the hardness & improve the surface texture

• Expensive

Tin (Sn) • Increase the tarnishing resistance

• Increase the hardness

Iridium (Ir) • As grain refiner

• Hardspot problem & expensive

Page 14: Seminar Heraeus

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Master Alloys vs Raw Metals

Master Alloys Raw Metals

1. The alloy is already in ready-

use form

2. Convenient and wide varieties

3. The property is well-studied

and the quality is stable

4. Special ingredients added to

modify the properties

5. Relatively expensive

1. The metals have to be weighed

and mixed prior to use

2. Inconvenient

3. The quality may not be stable

unless proper QC is done

4. Properties may not be studied

well

5. Cheaper

Page 15: Seminar Heraeus

Jewelry Production by Loss

Wax Casting

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Loss Wax Casting

Widely applied for solid jewelry

production

Stone in place casting is possible

Suitable for mass production

Flexible

Less machines involved

Normally more labor intensive

Alloys for this process is not

necessary to be very ductile

Page 17: Seminar Heraeus

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Major Steps in Loss Wax Casting

Model Design Rubber Mold Making Wax

Models

Treeing

Investment

Flask MakingBurn-outCasting

Completion &

Post-Treatment

Wax Setting

Page 18: Seminar Heraeus

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Examples of Jewelry by Loss Wax Casting

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What are the Major Factors for Casting?

Sprue Design

Melting/Casting Methods

Operation Temperatures

The Choice of Alloys and Alloy Quality

Page 20: Seminar Heraeus

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Rule of Thumb for Sprue Design

Sprue just resembles the blood vessels in our body

Main Sprue > Branch Sprue > Feed Sprue

Connect to the thickest part of the cast.

Prefer in fluted shape.

Extra sprue needed for heavy parts or items with large surface area

Avoid the sudden turning sprue to minimize turbulence

Page 21: Seminar Heraeus

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Opening of the Feed Sprue

Tapered sprue needs longer time for form-filling

Turbulence generates by the jet-like feeding.

The slightly fluted opening allows laminar flow of metals

Normal Tapered

Page 22: Seminar Heraeus

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Sprue Position

5-6% shrinking in solidifcation of metals

Solidified sprue will shut the opening Shrinkage porosity

Shrinkage Porosity

The sprue attaches to the thickest part of the cast.

The thickest part has to be hollowed if no attachment of sprue is allowed

Page 23: Seminar Heraeus

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How to Check the Sprue?

Experience is critical & important

Different trials can be used in early stage

Different designs prefered to be separated in different trees.

A tracable record for particular designs is suggested.

Trial 1

Trial 2

Trial 3Trial 4

Best Result

Page 24: Seminar Heraeus

Page 24

Melting and Casting Methods

Torch Melting

Resistance Melting

Induction Melting

Page 25: Seminar Heraeus

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Torch Melting

Fast, convenient and cheap

Use only natural gas, LPG with

compressed air

Never use acetylene as it gives

oxidizing flame

The flame must always cover

the melt during melting

Causes oxidation problem

Not-consistent in mass-

production

Page 26: Seminar Heraeus

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Resistance MeltingHeat generated from electric resistance

Longer heating time required ( higher risk

in oxidation)

Max. temperature of only 1100°C

No stirring effect Inhomogenous

Page 27: Seminar Heraeus

Page 27

Induction MeltingThe most advanced heating method

With stirring effect

No protection from oxidation

Fast & convenient

Relatively expensive

Page 28: Seminar Heraeus

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Vacuum Assisted Casting vs. Centrifugal Casting

Vacuum Assisted Casting Centrifugal Casting

- By weight & pressure difference - By centrifugal force

- For alloys with lower melt temp. - For alloys with high melt temp.

- Good for cast w large surface area - Surface texture hard to control

- More suitable for mass production - Good for filigree items

- Requires higher flask temp. - Requires lower flask temp.

Page 29: Seminar Heraeus

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How Operational Temperatures Affect Casting?

Casting and flask temperatures are critical for Casting quality.

The casting temperature controls the fluidity of the alloys, whilst the

flask temperature affects the texture of the cast surface

Studies showed that casting result is more sensitive towards the flask

temperature

- Too low flask temperature incomplete form-filling.

- Too high flask temperature dendritic surface & gas porosity

Page 30: Seminar Heraeus

Page 30

Gypsum decomposes over 730°C

Page 31: Seminar Heraeus

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Dendritic Surface Formed from High Temperature

Dendritic

Surface

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How Investment Burnt-Out Affects Casting?

In burnt-out cycle, the wax will be burnt off in form of CO2.

The residual carbon will react with metals or oxide to form CO2

which will cause serious surface defects.

Important

Page 34: Seminar Heraeus

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Investment After Burnt-Out

Incomplete Burnt-out Complete Burnt-out

Carbon

soot

Page 35: Seminar Heraeus

Jewelry Production by

Machining

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Machining By applying various deformation processes

(e.g. Stamping, wiring, rolling, drawing, tubing,

CNC cutting), the alloy is deformed to desired

shape to make the jewelry.

Highly automatic, machinery dependent and

relatively less labour demanding.

The unit production cost is low if large volume

is made.

Only suitable for big quantity production.

Page 37: Seminar Heraeus

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Examples of Jewelry by Machining or Handmaking

Page 39: Seminar Heraeus

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Major Factors affecting Machining Performance

Annealing parameters

Alloy Quality

Etching

Oxidation

Equipment Quality

Page 40: Seminar Heraeus

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Annealing

To restore the ductility of the

alloy

Achieved by annealing

furnace or torch

Induced by recrystalizing the

metal atoms

Over-annealing excess

crystal growth peeling &

cracking

The alloy must be covered

by protective gases (e.g.

cracked ammonia)

Page 41: Seminar Heraeus

Page 41

Technical Seminar in Iran 2007

Annealing Process

Highly deformed crystals

Recrystalization

Crystals start

to grow

High-Lights of Jewelry Production

Page 42: Seminar Heraeus

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Suggestions for AnnealingAlloy Type Annealing

Temp (°C)

Temperature

Color

Reduction

18-22KY 550 – 600 Very Dark Red 70 – 80 %

8-14KY 650 – 670 Dark Red 60 – 70 %

8-18KR 650 – 680 Cherry Red 60 – 70%

8-18KW- Ni 650 – 700 Cherry Red 50 – 60 %

925Ag 550 – 600 Very Dark Red 70 – 80 %

Sheet

CW% = Int. Thick (to) – Fin. Thick(t1)

Int. Thick (to)

Wire

CW% = Int. Area (ro2) – Fin. Area (r12)

Int. Area (ro2)

Page 43: Seminar Heraeus

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Why Finer Grains Better?Metal is polycrystalline structured

The lines between grains are called

„grain boundary“

The boundary is the line of weakness

and site for impurities accumulation

Finer grains more boundary for

impurities. The alloy is less sensitive

towards contaminations

Easier to polish & more lustrous

Larger grains deformed differently to

accomodate the shape changes

rough surface & peeling effect

Large Grains

Finer Grains

Page 44: Seminar Heraeus

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Consequences of Improper Annealing ProcessToo low the annealing temperature and/or time cannot induce the recrystallization.

The alloy remains hard and not ductile.

Too high annealing temperature and/or time enhances the crystal growth

resulting large grains. It causes more peeling problem and sometimes cracking

problem.

No protective coverage in annealing will cause severe oxidation to the alloys

resulting discoloration, embrittlement and blistering in the alloy.

Too frequent annealing is also not good. It enhances the extensive crystal growth

resulting peeling problem afterwards.

Page 45: Seminar Heraeus

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Major Criteria of the Alloys for Machining

Alloy can be produced by continuous casting and static ingot

casting

The alloy cannot be casted with too high temperature. It would

give large pipe in ingot and cause excessive grain growth

The alloys for investment casting are not suggested for machining

purpose due to the presence of silicon or the investment inclusion

White gold should not have too much nickel, otherwise it becomes

brittle due to nickel segregation.

Prior to recycling the alloy, the oily substance must be removed by

detergent or petroleum

The heavily recycled alloy should be avoided due to the content of

unknown impurities

Page 46: Seminar Heraeus

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Major Criteria of the Alloys for Machining

Au + Ni

Segregation of Ni

Page 47: Seminar Heraeus

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Etching

Etching is a process to remove the core

The etching process depends on the cores used:

- Copper/Tombac (Cu-Zn alloy) Nitric acid

- Iron hydrochloric acid or dilute Sulfuric acid

- Aluminum Caustic Soda (sodium Hydroxide)

Improper etching will dissolve the gold alloy or attack the grain

boundary rendering brittleness in the items

Complete removal of core is essential in order to meet the

hallmarking.

Page 48: Seminar Heraeus

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Oxidation

Oxidation happens in casting and

annealing without covering gases.

The presence of oxide can cause:

- reduction of ductility

- Formation of blisters

- Formation of porosity

- Color change

The alloy must be covered with

protective gas whenever heat

applies

Page 49: Seminar Heraeus

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Equipment QualityMachining performance highly

depends on the quality of equipment

- Alignment of the rolls

- Surface texture of the rolls

- The proper procedure for the machine operation

Page 50: Seminar Heraeus

Common Defects in Jewelry

Production

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Why Gas Porosity?It looks like small spherical pores

on the surfaces (pin-hole) & difficult

to be removed by polishing

Caused by the gaseous

contaminants (e.g. O2 or SO2)

High operation temperature causes

the decomposition of gypsum (i.e.

CaSO4)

CaSO4 CaO + ½ O2 + SO2

Heavily recycled metals contain

large amount of gaseous

contaminants from gypsum or

copper oxide

Page 52: Seminar Heraeus

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Oxidative Inclusion Porosity“Crow-like“ porosity on the surface &

the region nearby may be brittle

It is more abundant in the alloy with

higher Zinc content (Ni-KW or 8-

14KY)

The zinc oxide is trapped along the

grain boundary and left the cavity

resembling shrinkage porosity

The problem is caused by severe

oxidation or extensive use of recycled

alloys

Page 53: Seminar Heraeus

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How the Shrinkage Porosity Forms?

- ZnO - Cu2O

Page 54: Seminar Heraeus

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Why We Add Zinc in Alloys?

Zinc is added to karat gold as a deoxidizer. It can reduce the copper

oxide back to metallic state with itself converted to zinc oxide.

Zn + Cu2O → ZnO + 2Cu

Zinc is added to yellow gold to enrich the yellow tints of the low karat

yellow gold alloys.

Zinc is added to Ni-white gold to increase the fluidity of the molten

alloys. It can reduce also the surface tension of the nickel-white gold.

The Zinc is easy to be oxidized to feathery Zinc oxide and trapped in

the alloys resulting oxidative shrinkage porosity.

It increases the melt loss.

Page 55: Seminar Heraeus

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Why Alloy Cracks?Some jewelry alloys are more sensitive towards cracking. It is due to

the inherent problems and the operational conditions.

Examples of alloys sentive towards cracking:

1. Alloy with high silicon content

2. 18K pink gold

3. 18K yellow gold with high zinc content

4. Ni-white gold with high nickel content

5. Heavily recycled alloys

Page 56: Seminar Heraeus

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Use of Silicon in Jewelry Alloys

The use of Silicon in jewelry casting are under debate for decades. The

table listed the pro and con of silicon in casting

Advantages Disadvantages

1. Deoxidizer to give shiny

surface on the cast

2. Increase the recycling time

3. Increase alloy hardness

4. Increase alloy fluidity

1. Favor large grains formation

2. Its segregation causes

brittleness

3. Causes hard-spot problem in

Ni-white gold

The more protection from oxidation during casting is, the less the

silicon.

Page 57: Seminar Heraeus

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Jewelry Alloys with Silicon

Si not soluble in Au & Ag

segregation

Adundant in 18KY and sterling silver

Si-rich

phase

Page 58: Seminar Heraeus

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Cracking in 18K Pink Gold

18K pink gold cracked like biscuit

upon slow cooling after heating

Due to the intermetallics formation

410ºC

Ordering

Ductile Brittle

Page 59: Seminar Heraeus

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Why 18KY gold cracks with 14KY alloy?

The high zinc content in 18KY makes it easier to crack.

Master for 14KY gold contains more zinc for bringing the yellow color.

Many different phases between Au & Zn Brittle

Page 60: Seminar Heraeus

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Peeling

Caused by the excessive growth

of crystal grains

The alloy easily peels

Factors favors large crystal grains

- High annealing temperature

- Long annealing time

- High casting temperature

- Alloy contains silicon

Page 61: Seminar Heraeus

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Hard Ni-white Gold = Easy to Crack?It is true that Ni-white gold is about 40 – 70% harder than the normal

yellow gold.

The cracking problem is due

to the segregation of nickel

from gold alloy.

Alloy with higher Cu content

or lower Ni content and fast

quenching can improve the

brittleness.

Au + Ni

Page 62: Seminar Heraeus

Page 62

Why Recycled Alloys Easy to Crack?

In recycling of alloys, impurities will

accumulate in the alloys.

The impurities segregate Cracks

Impruities = oxides, investment

residue, iron from tools & polishing

media

O (ppm) 14KY Ni-14KW

1st melt 20.2 16.5

2nd melt 13.0 46.1

3rd melt 20.2 72.2

4th melt 26.1 119.0

5th melt 90.0 178.7

Page 63: Seminar Heraeus

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Where the Hard Spot Come From?Hard spot is not found during filing. It is

often found during the polishing in the

later stage.

The hard spot has higher hardness than

the bulk alloys. It resembles the stones

on the muddy road.

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Hard Spots in Ni-KW GoldCommonly found in Ni-alloy with silicon

Appears only on one side of the item

(Due to the different densities from gold)

Possible Cause:

- The formation of nickel Silicide

- The segregation of Si in Nickel

Page 65: Seminar Heraeus

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Why Ni & Si are present Together?

Reaction Between Ni & Si

- Favored by carbon from crucible and /or incomplete burnt wax

- Favored by heavily recycled alloys

- High Operational Temp

• Si dissolves in Ni

- Silicon has good solubility

nickel

- Dissolved in segregated

nickel

Page 66: Seminar Heraeus

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Hard Spot from GoldHard spot all over the surface

Originated from pure goldTungsten (W)

Ru, Ir, Os

Page 67: Seminar Heraeus

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Why Fire-Scale Happens?

Red-stain

Red-stain

After polishing, colored stains

appeared on sterling silver, removed

by intensive polishing or acid pickling

not possible

Caused by the formation of copper

oxides (i.e. Cu2O or CuO)

Cu2O

Stained

Area

Normal

Area

Page 68: Seminar Heraeus

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Cross-section of Red-Stain AreaThe stain is not only on the surface but penetrated also to sub-surface

Red-stain

Page 69: Seminar Heraeus

Recycling of Scraps

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What are the Common Impurities in Scraps?

From casting, the scrap usually contains investment residues and

oxides.

From machining, the scrap contains usually gease, steel residues and

oxides.

The investment residue will cause hard-spot and gas porosity problem.

The impurities are usually present on the surface only unless the

melting is not handled properly with serious oxidation.

Page 71: Seminar Heraeus

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Why We Need to Add New Alloy in each Cast?

There are some additives in the alloy and they may burn off or

evaporate during the melting process. The purpose of adding the new

material is to replenish the additives.

E.g. 30% of Additive A will evaporate during melting.

Initial: 1% Final: 0.7%

I. If only 100% of old material is used, only 0.7% of the additive A in

the alloy. The performance of the alloy may vary.

II. If 50% of new material is added, the additive A level will be:

0.7% x 0.5 + 1% x 0.5 = 0.85%

The change will be less significant and the performance and

quality of the alloy can be maintained.

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How Can We Clean the Scrap?

The scrap has to cut into pieces to have visual inspection whether it is

heavily contaminated. The heavily contaminated parts have to be

refined.

The scrap is put in tumbler to clean the surface mechanically. The

oxide and investment can be removed effectively.

For machined scrap, the scrap have to be washed with detergent or

petroleum to remove all the geasy impurities.

The scrap is pickled in 15 – 20% dilute sulfuric acid to remove the

oxide.

The scrap is rinsed with water and dried. The scrap is clean to be

recycling.

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Flowchart of the Cleaning Process

Scrap Casting

Scrap?

Visually

clean?RefiningYes No

Pickle in

detergent/

gasoline

Tumbling

for 30 mins

Pickle in 20%

sulfuric acid

(10mins)

Rinse

with Water

Scrap clean

to be used

No Yes

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Inspect the Quality of the Scrap Visually!!

Cut scraps into pieces to inspect visually whether it is heavily

contaminated. The heavily contaminated parts have to be refined.

The cut edge showed significant deformation indicating the

contamination is limited.

Deformed surface Cut edge

High-Lights of Jewelry Production

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Clean the Scrap Prior to Re-use?

The scrap should be tumbled in tumbler

to clean the surface mechanically. The

oxide and investment machined scrap,

For the scrap from machining, it has to

be washed with detergent or petroleum

to remove all the geasy impurities.

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Pickle the Scrap to Remove Oxides

The scrap should be pickled in 15-

20% sulfuric acid to remove the

oxide on the surface.

The acid can only dissolve the

oxides but it will not attack the alloys.

The acid can be used repeatingly

and it is non-volatile.

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Scrap Ready to Use

After pickling, the scrap should be

rinsed with plenty of water to

remove the excess sulfate ions.

The scrap is dried in oven and it is

ready to be used.

The scrap can be used directly or

granulating prior to use.

Page 78: Seminar Heraeus

Metal Loss in Jewelry

Production

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Where the Metals Lost in the Production?

There are many ways to cause the metal loss during the production.

- Loss from melting or casting

- Loss from machining

- Loss from diamond cutting or polishing

- Loss from surface finishing

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Loss from Melting or Casting

Gold is stable towards oxidation and its vapor pressure is very low and

it will not evaporate easily.

Karat gold contains not only gold but also other metals such as silver,

copper, zinc or nickel. However, these metals can easily be oxidized

to their corresponding oxides. The oxides will then be etched away.

The melt loss is in fact not due to the loss of gold but the loss of

master alloys. You give more gold to your customer.

Way to avoid the high melting or casting loss

- Use protective gas to cover the melt to avoid the oxidation of alloys.

- To avoid the high operational temperature, the evaporation of the

alloys can be reduced.

- To avoid using alloys with volatile elements such as Zn or Si.

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Loss from Machining

The production loss from machining should be limited.

The loss usually goes to the peeling of metals

The loss can be minimized by improving the annealing condition.

The smaller the grain size, the less the peeling problem is.

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Loss from Diamond Cutting or Polishing

This loss is unavoidable but it can be minimized.

If the surface is rough, more intensive polihing has to be made and the

polishing loss will increase.

If the metal is too soft, it can also increase the polishing loss.

To minimize this loss, the polishing has to be kept minimal by carefully

adjust the operational temperature in casting or annealing.

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Loss from Surface Finishing

The surface will sometimes be treated before electroplating.

The bombing solution (H2O2 + cyanide solution) can help to make the

surface shiny. However, it can etch away the metal on the surface

and increase the metal loss.

To avoid using this method in cleansing process.

Page 84: Seminar Heraeus

New Technology in Jewelry

Production

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Hard Gold or Karat-Gold Electro-forming

To produce hollow jewelry, it involves many steps in metal

deformation. It can only be economical in production if the production

quantity is large.

Electro-forming can shorten the production period and bigger variety

in the design can be achieved.

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Process for Gold Electro-formingWe will first design the wax models

Followed by a metal plating on the wax master or put on silver paste

layer.

Conduct the electroforming process

Remove the wax and the base metal layer

Page 87: Seminar Heraeus

How Heraeus helps the

Jewelry Industry?

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How can Heraeus Help?

Heraeus Group was established in Hanau Germany in 1851. We have

over 150 years of experience in precious metal technology

Functional Materials Unit (FM), formerly known as Semi-finished Jewelry

Division (SJD) has been supplying master alloys and semi-finished

jewelry products for decades.

Heraeus has the alloy production in Hong Kong to give the fastest and

best service to jewelry manufacturers in Asia. Our R&D laboratory in HK

also keeps develop newer and better products for the industry.

Heraeus Indonesia & PT. Hemas are our partners for master alloys in

Indonesia. They can give you almost the instant response whenever our

clients have any requests.

Page 89: Seminar Heraeus

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R&D Laboratory in Heraeus Ltd.

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Some Popular Master Alloys from Heraeus

Master

Alloy

Application Advantages

H-128I 10-18K Pink It gives nice pinkish yellow color & easy to cast with

high recycling rate

H-147A 14-18KW It gives brilliant white color and good for casting

H-148A 14-18KW It is very ductile and perfect for machining purpose

H-449 Sterling silver It gives very white color and easy to cast with. No

firescales and no plating needed

H-PAT Sterling silver It gives very white and smooth surface after casting

Page 91: Seminar Heraeus

Page 91Heraeus Ltd., Dr. P. H. Ko, SJD, Iran Jul 2010

Thanks for Your

Attention !