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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
Metals for Jewelry
Productions
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
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
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.
Basic Metallurgy
Page 7
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
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
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
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.
Why Master Alloys ?
Page 12
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
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
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
Jewelry Production by Loss
Wax Casting
Page 16
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
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
Examples of Jewelry by Loss Wax Casting
Page 19
What are the Major Factors for Casting?
Sprue Design
Melting/Casting Methods
Operation Temperatures
The Choice of Alloys and Alloy Quality
Page 20
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
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
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
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
Melting and Casting Methods
Torch Melting
Resistance Melting
Induction Melting
Page 25
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
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
Induction MeltingThe most advanced heating method
With stirring effect
No protection from oxidation
Fast & convenient
Relatively expensive
Page 28
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
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
Gypsum decomposes over 730°C
Page 31
Dendritic Surface Formed from High Temperature
Dendritic
Surface
Page 32
Examples of Flask TemperaturesFlask Temp Items Examples
< 520 ºC Heavy
Bracelets
510 – 550 ºC Men’s Ring
540 – 600 ºC Women’s
Ring
580 – 630 ºC Filigree Items
Page 33
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
Investment After Burnt-Out
Incomplete Burnt-out Complete Burnt-out
Carbon
soot
Jewelry Production by
Machining
Page 36
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
Examples of Jewelry by Machining or Handmaking
Page 38
Major Steps in Machining
Preparation ofMetal Ingots
Wiring
Solid Chains
Profiling
Tubing
Stamping
Rolling
DiamondCutting
EtchingChain Knitting
Hollow Jewelry
Stamping
Rolling
Findings
Page 39
Major Factors affecting Machining Performance
Annealing parameters
Alloy Quality
Etching
Oxidation
Equipment Quality
Page 40
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
Technical Seminar in Iran 2007
Annealing Process
Highly deformed crystals
Recrystalization
Crystals start
to grow
High-Lights of Jewelry Production
Page 42
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
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
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
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
Major Criteria of the Alloys for Machining
Au + Ni
Segregation of Ni
Page 47
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
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
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
Common Defects in Jewelry
Production
Page 51
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
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
How the Shrinkage Porosity Forms?
- ZnO - Cu2O
Page 54
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
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
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
Jewelry Alloys with Silicon
Si not soluble in Au & Ag
segregation
Adundant in 18KY and sterling silver
Si-rich
phase
Page 58
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
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
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
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
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
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.
Page 64
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
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
Hard Spot from GoldHard spot all over the surface
Originated from pure goldTungsten (W)
Ru, Ir, Os
Page 67
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
Cross-section of Red-Stain AreaThe stain is not only on the surface but penetrated also to sub-surface
Red-stain
Recycling of Scraps
Page 70
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
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.
Page 72
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.
Page 73
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
Page 74
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
Page 75
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.
Page 76
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.
Page 77
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.
Metal Loss in Jewelry
Production
Page 79
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
Page 80
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.
Page 81
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.
Page 82
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.
Page 83
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.
New Technology in Jewelry
Production
Page 85
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.
Page 86
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
How Heraeus helps the
Jewelry Industry?
Page 88
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
R&D Laboratory in Heraeus Ltd.
Page 90
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 91Heraeus Ltd., Dr. P. H. Ko, SJD, Iran Jul 2010
Thanks for Your
Attention !
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