Welcome to The Learning Lounge our Glossary of Antique & Modern Jewellery Terms, Techniques & Styles
A gold coloured alloy of copper 83% and zinc 17%. and less dense than gold. It was used in the manufacture of jewellery and other objects, invented by London watchmaker Christopher Pinchbeck in circa 1720. With the advent of the middle class due to the Industrial Revolution a niche market was created for people who desired beautifully made items at an affordable price. The technique was continued by his son until 1835 when electroplating was introduced. It was used extensively in the manufacture of utility items such as chains, watch cases, chatelaines, and buckles. Pinchbeck is very collectible ecause the skills and the attention to detail on the items produced was the same as if the item was produced in gold. Because Pinchbeck is a solid metal it can be polished without losing its colour. Though no pieces of Pinchbeck were hallmarked due to the fact that it was only produced for 100 years it is easily date
Old Sheffield Plate:
Old Sheffield Plate is the term used to denote articles made of copper with a thin silver by fused to the copper. This process, which was invented Thomas Boulsover in 1742, ceased following the introduction of much cheaper silver-plating by electro-deposition (EPNS) about the middle of the 19th century. Old Sheffield plate was used in the production of hollow ware and flat ware. It can be distinguished from the later EPNS items because when the sheet was rolled it was soldered together with silver. The soldering line is visible. Also where the leading wedge of the sheet is visible a border of finely pressed sterling silver was soldered on to hide the copper. In later pieces an engraving plate was also set into the body of the item so that when an engraving was applied no copper would be visible. Items of Old Sheffield plate should be cleaned with care because excessive rubbing will eventually expose the underlying copper layer. In Old Sheffield plate connoisseurs circles exposed copper is not an issue. This adds character to the item. It is highly recommended that the item not be silver plated. Unlike EPNS, Old Sheffield plate used pseudo hallmark punches and makers marks on the items which have been well documented and makes them easy to date.
Electroplated nickel silver or E.P.N.S. was introduced c.1845. It was a process of making articles with a nickel-silver base alloy and then silver plating them. Because the base metal of E.P.N.S. had a similar colour to silver, this processes became more popular than use of a copper base for Old Sheffield Plate which tended to show its coppery brown base colour as its silver coating wore off.
Electroplated Britannia metal (E.P.B.M.) was introduced c.1875 and was considered to be the poor man’s silver ware. The base metal was Britannia metal, which was an alloy of lead and tin closely allied to pewter. E.P. B.M. has one great disadvantage. It is very susceptible to damage by applied heat. Consequently, if an article of E.P.B.M. is brought in for repair, it will not take any applied heat without damage.
The Pearl Classification System:
Pearls are classified by their type, size, shape, lustre, nacre finish and colour.
A natural pearl is a pearl produced by a mollusc as a natures way of reducing an irritant that has penetrated through the shell or body of the mollusc. All bivalve molluscs can produce pearls, however only a few varieties produce what we call gem quality pearls.
The Akoya oyster produces what is called the "Japanese" cultured pearl. Though the culturing process was not invented by Mikimoto, he was the Barnam and Bailey of the pearling industry and created the market for teh cultured pearl. The sizes range from the tiny seed pearls to up to 10mm. in diameter.
South Sea Pearl:
The oyster variety Pintarda Maxima produces the largest pearls, in an array of hues which are known as "South Sea Pearls". Australia is famed for producing some of the world's finest South Sea Pearls. The pearls generally range from 8mm to about 17mm in diameter, though they have been known to reach up to 20mm.
Fresh water pearls are produced by mussels. Originally looked upon as the poorer cousin to salt water pearls, the techniques being used and developed in the mussel culturing process have made the the difference very difficult to detect. Fresh water pearls come in a variety of shapes, colours and sizes.
The last cycle of the culturing process is the making of the mabe or blister pearl. This is achieved by gluing a dome, tear drop or other shape to the inside of the shell of the mollusc and allowing the oyster to coat the implanted shape with nacre.
The Black Pearl:
The queen of the sea. The black pearl is produced by a special variety of a large oyster known as the "Black Lipped Oyster". Due to its rarity, French Polynesia and the Cook Islands have jealously guarded these oysters. The sizes achieved are similar to the Pintarda Maxima or "South Sea Oyster". Black pearls are classified in the same way as other pearls.
No other issue in the pearling world is more hotly debated than the classification of the Keshi pearl. Keshi pearls are produced as a by-product of the culturing process. They are produced by the oyster rejecting the implanted nucleus and organicaly producing a pearl of its own. Keshi pearls are ver irregular in shape.
In Pearls, size does matter. It is one of the fundamental criteria of how a pearl is classified and valued.
Uniform round: The Pearl should be extremely uniform in shape and roundness.
Semi round: The pearl should be only slightly off round.
Semi baroque: A pearl with a distinct off-round to irregular shape which is easily recognisable to the eye.
Baroque: A pearl with a highly irregular shape. It may elongated and show what is known as a "tail".
Teardrop: A pearl which is in the shape of a tear drop.
Button: A pearl with a flattened, button like profile.
Luster is dependant on the quality of the oyster, how well it has created the nacre of the pearl and how long it has been given to do so. The "orient" of the pearl, which gives it it's luster is due to the refraction of light on the nacre and gives the sheen and hue which is so sought after in pearls.
Pearls come in a variety of colours. Some occur naturally while others are treated. White pearls occur in various shades of silver and cream. Golden pearls fetch a premium price for their warm hue. Black and silver pearls are created by the black lip oyster and are often wound with a secondary overtone of aubergine or peacock green.
Opal is hydrated amorphous silica (SiO2.nH2O), there are
two basic forms of opal described by visual appearance:
The Gemmological Association of Australia (GAA) Opal Classification system.
In 1991 The GAA was asked to produce a classification system for opal our national gemstone. The resulting system has been adopted world wide by the world jewellery congress (CIBJO).
Opal which exhibits the phenomena known as “play of colour” and is produced
by diffraction light due a micro-structure of orderly
arranged silica spheres.
Common or Potch Opal:
Opal which does not show a “play of colour”. The distinction between common opal and potch opal is based on formation and structure. Potch opal is structurally similar to precious opal but has a disorderly sphere arrangement. Common opal shows some degree of micro crystallinity.
Types of Natural Opal
Opal presented in one piece in its natural state other that cutting or polishing and encompasses all opal of the same state or structure and of substantially homogenous chemical composition.
Opal presented in one piece where the opal is naturally attached to the host rock in which it was formed where the host rock is of a different chemical composition and is known as "Boulder Opal".
Opal presented in one piece where the opal is intimately diffused as infillings of pores or holes or between grains of the host rock in which it was formed and is known as "Matrix Opal".
Varieties of Natural Opal:
The variety of natural opal is determined by the two characteristics of body tone and transparency.
The body tone of an opal is different to the "play of colour" displayed in precious opals. There are three varieties of natural opal based on body tone. Body tone refers to the relative darkness or lightness of the opal when ignoring the "play of colour".
Black opal is the family of precious opal which shows a "play of colour" within or on a black or dark grey body colour when viewed face up and may be designated N1, N2, N3 or N4. (see table below).
Dark opalis the family of opal which shows "play of colour"within or on a dark or medium dark body
tone when viewed face up and may be designated N5 or N6. (see table below)
Light opalis the family of opal which shows a "play of colour"within or on a light body when viewed face up and may be designated N7, N8 or N9. The category N9 is referred to as white opal. (see table below).
The Body Tone Scale:
The boxes below represent approximate values of body tone in equal 10% intervals from black to white. This is in agreement with all known colour science tables for tone. It is illustrated in the commercially available Geological Society of America. The A.G.I.A. body tone scale is being developed using computer graphics and when available will correlate with these charts.
Opal shows all forms of diaphaneity and ranges from transparent to opaque. Precious opals which are
transparent through to semi-transparent are known as crystal opal. Crystal opals can also have either a black, dark or light body tone. The term “crystal” in this context refers to appearance not a crystalline structure.
Opal can be subjected to various types of treatment. Present CIBJO guidelines state that any method of treatment other than standard cutting and polishing must be disclosed and the process used specified on all invoices, advertising and commercial documents. Types of treatments include colour enhancement, heating, painting, dying, resins, waxes, oiling or durability (see discussion on composite opals.) Opal body tone is often enhanced (usually altered to appear darker). In some opal inlay jewellery where usually a thin piece of crystal opal is painted black or black glue for example is used to cement the opal in jewellery.
COMPOSITE NATURAL OPAL
Doublet Opals are composites consisting of two pieces where a slice of natural opal is cemented onto a dark base material.
Triplet Opal are a composition of three pieces where a thin slice of natural opal is cemented to a dark base material and a transparent top layer usually glass or quartz.
Mosaic Opal & Chip Opals are a composition of small flat or irregularly shaped natural opal pieces cemented as a mosaic tile on a dark base material encompassed in a resin.
OTHER CATEGORIES OF OPAL
Synthetic opal is material that has essentially the same chemical composition and physical structure as natural opal but has been made by laboratory or industrial process. Synthetic composites exist as synthetic doublets, triplets or mosaics and must be disclosed as synthetic opal composites.
Imitation Opal is material which imitates the“play of colour” of natural opal, but does not
have the same physical and chemical structure or
gemmological constants as natural opal.
Classification reports for the following types of opal should include these details:-
1. Type of opal
2. Variety of opal as Black opal, Dark opal or Light opal with a body classification from N1 (Black) to N9 (White) based on the Body Tone Chart.
3. Transparency opaque, translucent or transparent. Note if the specimen is crystal opal.
4. Type of treatment and process if known.
5. Weight and Dimensions
1. Type of composite as doublet, triplet, mosaic, or chip opal
2. Treatment process, where relevant
Synthetic and Imitation
1. Gemmological category including manufacturer (if Known)
2. Description (Body Tone)
3. If Composite, mention type as doublet, triplet, mosaic or chip
4. Weight and dimensions, only if composite.
Any indication of the origin of opal by the use of geographical location should not be used unless it is qualified as an indication of the TYPE of locality only as recommended by C.I.B.J.O. (e.g. Lightning Ridge type black opal).