Guide · 7 minute read
Dental gold: how crowns, bridges and inlays are tested
The short answer
Nobody can tell what a crown holds by looking at it. Dental alloys vary far more than jewellery does: some crowns are mostly gold with platinum-group metals, some hold a modest amount, and some are grey base alloys with no precious metal at all. Only a test on exposed metal separates them, and on a small, curved crown even a good test has limits. Porcelain, tooth, cement and base-metal posts are not metal, so their weight is estimated and left out. None of it is chipped off or removed before you have accepted an offer, and no spot is cleaned without your written consent (clauses 4.2 and 4.4 of the terms). Every piece gets its own line, and one written offer for the lot follows.
No obligation. Decline and it all comes back free; ask for part of it back and a fresh offer is made for the rest.
What dental gold is and how the alloys differ
Dental gold is the metal in crowns, bridges, inlays, onlays and some denture clasps and frameworks. It is never pure. Laboratories choose alloys for hardness, casting and resistance to corrosion in the mouth, and the recipe changes between laboratories, between decades and between types of restoration. Dental work carries no hallmark and no fineness stamp, so nothing on the piece tells you what it holds until it is tested.
- High-noble alloy
- Gold and the platinum-group metals make up most of the metal, with gold the largest single part. These are the pieces most worth sending, and the gold share still has to be measured on each one.
- Noble alloy
- A meaningful but smaller share of gold, palladium or platinum, balanced by silver, copper and other metals. Palladium-based crowns sit here and can hold very little gold.
- Base alloy
- Nickel-chromium or cobalt-chromium, with no gold in it. It looks grey rather than yellow, and there is no precious metal in it at all.
- Porcelain-fused-to-metal (PFM)
- A crown with a metal core under a ceramic layer. The ceramic weighs a great deal compared with the metal, and none of it is metal.
Because the gold share of a high-noble alloy differs from one crown to the next, Assaypost does not quote a typical dental fineness, and does not take one on trust from a seller or a dentist. The dental gold page lists what is taken and what is not, including amalgam fillings, which contain mercury and are not handled at all.
What on a crown is not metal
A crown as it comes out of the mouth, or as a dentist hands it back in an envelope, is rarely just metal. Each part that is not metal has to be accounted for before anyone can say what the piece is. None of it is taken off before you have accepted an offer.
- Fragments of natural tooth are often still cemented inside a crown. Their weight is estimated and left out.
- On a porcelain-fused-to-metal crown the ceramic can outweigh the metal. It stays on, and the metal under it is estimated, with the line saying so.
- Cement leaves a thin film that adds weight and blocks the surface a test needs to read. Cleaning a spot to read it needs your written consent (clause 4.2).
- Stainless or nickel-alloy posts and pins inside a crown are base metal, and they distort a reading if they sit in the test area.
- Clasps and frameworks from partial dentures stay in the pink acrylic, and the weight of the acrylic is estimated and left out.
Everything left out appears on the item's line: what it is, how much of it there was taken to be and how that was estimated. The same rule applies to the rest of a parcel, and the guide to stones, clasps and what is not metal covers it.
Hygiene, and what happens to a crown when it arrives
Dental work has been in a mouth. Rinse each piece in warm water, dry it and seal it in a small zip bag before packing. Do not try to remove porcelain or tooth yourself: it needs the right tools and a way to catch the fragments, and it does not need doing before an offer.
- Weighed as it arrivedEach piece is weighed alone on a balance reading to 0.01 g. What is taken to be tooth, cement or ceramic is estimated, left out and written on the line as an estimate.
- Nothing taken offNothing is cleaned, chipped or scraped. Cement, tooth and ceramic stay where they are, unless you agree in writing to one small spot being cleaned so that it can be read (clause 4.2).
- TestedEach piece, or each group of like pieces, is tested as the next section describes, and every result goes on its own line with the place it was taken.
How dental gold is tested, and why XRF struggles with small pieces
Where an analyser is available, X-ray fluorescence (XRF) is the natural first test. No analyser has been named yet, so it is not a check you can count on (clause 4.1 of the terms). XRF is non-destructive and reads the composition of the surface, typically to a depth of tens of microns. On a flat, clean, uniform sample it gives a dependable reading. A crown is none of those things.
- A crown may offer only a few square millimetres of clean metal, and small or thin samples give readings with wider uncertainty than a bar or a chain.
- XRF assumes the sample fills the measurement window. A curved crown wall leaves gaps, and gaps weaken the signal and can bias the result.
- Years in the mouth can leave an oxidised or enriched surface layer, and cement blocks the reading altogether, so a surface result may not match the alloy beneath.
- Bridges are joined with solders of a different composition. A reading taken on a joint reports the solder, not the crown.
- Metal under a ceramic layer cannot be read through it. Only exposed metal is tested.
So a crown is tested in more than one place where the piece allows, every test location is written down, and the fineness concluded is recorded beside the limits of the result behind it. Density is not used on porcelain-bonded pieces, because the bonded ceramic makes the result meaningless.
Where results disagree, the method that would settle it is an assay of the melted metal. No arrangement with a laboratory exists, so Assaypost cannot propose one. A piece that stays unsettled is either priced conservatively or left out of the offer, and the offer says which (clause 4.4). The XRF guide and the testing page go further into the method.
What a test settles on a crown, and what it leaves open
A test on a crown does one job well: it says which of the three families a piece belongs to. That is less than sellers expect, and more than guessing gives them.
What the checks on a crown establish
| Check | What it settles | What it still leaves open |
|---|---|---|
| Magnet | A piece that pulls has steel in it, usually a post or a pin | Many base dental alloys do not pull, so a crown that ignores a magnet can still hold no precious metal |
| Weight, less the estimated tooth and cement | Roughly how much of what arrived is metal | Nothing about which metals they are |
| XRF on exposed metal | Which elements are present, and roughly in what proportion | Whether the surface represents the body, on an oxidised or soldered piece |
| A second and third reading | Whether the first was representative or sat on a joint | The metal under a bonded ceramic layer, which cannot be read through |
| Assay of melted metal | The composition exactly, for a larger lot | Nothing, but no arrangement with a laboratory exists, so it cannot be offered |
The practical point for a seller: two crowns from the same mouth in the same decade can be a high-noble alloy and a base alloy, and look alike in an envelope. Anything said about them before they are tested describes the envelope, not the crowns.
It is also why dental work belongs in a parcel with everything else. A handful of crowns is small, awkward and hard to judge on its own. Sent with a box of jewellery, it gets the same bench and the same written list, and one offer covers the lot.
Seller risks and exceptions
- Assuming a fineness. One figure for all dental gold, given without testing, is a guess or an average, and an average means the high-noble crowns pay for the base-alloy ones. Ask for the result on each piece.
- Weighing with the tooth still in. Tooth and cement can add a good deal to a crown's weight. Any weight that does not allow for them includes the tooth.
- Palladium-based crowns. Some noble alloys are mostly palladium with very little gold. Palladium is a precious metal in its own right, and the line for such a crown says which metal it is. See the platinum and palladium page.
- Base-alloy crowns. Grey crowns in nickel-chromium or cobalt-chromium hold no precious metal. They are still listed, so that you can see they arrived (clause 4.4), and they come back to you.
- Very small pieces. A single inlay may weigh under half a gram. It is examined and tested the same way, but the result carries more uncertainty at that size, and the line says so.
- Amalgam. Silver-coloured fillings contain mercury, and amalgam is not taken, tested or handled, so leave it out of the parcel. The law requires dental practices to send amalgam waste, including teeth contaminated with it, to an authorised waste handler (Article 10(6) of the retained Mercury Regulation).
When not to use the scrap route
Dental gold rarely has a value above its metal, with two exceptions
Gold teeth set into antique or ethnographic jewellery belong to that object and should be assessed as jewellery. And a large quantity, such as a dental laboratory clear-out, is better settled by an assay of the melted metal than by a test on each piece. No arrangement with a laboratory exists, so that is not a route Assaypost can offer, and a lot that size may do better with a refiner who can.
For ordinary crowns and bridges from one person's dental history, the question is whether whoever receives them tests each piece and shows you the result. A photograph cannot establish a dental alloy, so this is one of the few things where posting is the only way to find out. Send the crowns with the rest of the box rather than on their own, and send photographs of the box first if you are unsure what else should go with them.
Questions sellers ask
What proportion of a dental crown is gold?
There is no single answer, and anyone who gives you one is guessing. High-noble crowns hold a majority of gold and platinum-group metals, noble alloys hold less, and base alloys hold none. The recipe differs from one laboratory to the next, and only a test on exposed metal comes close to settling it for a particular piece, with the limits of a surface reading stated beside it.
Do I need to get the tooth out of the crown before sending it?
No, and please do not try. Rinse it, dry it and seal it in a small bag. Tooth, cement and porcelain stay where they are until you have accepted an offer. Their weight is estimated and written on the line for that piece, and they come off afterwards, with the right tools.
Is a porcelain crown worth anything?
Only the metal core, if it has one. An all-ceramic crown has no metal in it at all. A porcelain-fused-to-metal crown has a metal core under the ceramic, and that core is what is assessed, with the ceramic left in place and its weight estimated.
Why can XRF give an uncertain reading on a crown?
XRF reads the surface to a depth of tens of microns and assumes a flat sample that fills its window. A crown is small, curved, sometimes oxidised after years in the mouth, and sometimes joined with solder of a different alloy. That is why a crown is tested in more than one place where it allows, with the limits of each result written beside it.
How do I know a gold-coloured crown is not a base alloy?
Often you cannot. Gold-coloured base alloys exist, and some noble alloys are pale. A magnet is little help, because many base dental alloys do not pull. A test on exposed metal settles it, and until a piece has been tested nothing about it is known.
Is it worth sending photographs of dental work first?
Yes, though expect less from them than for jewellery. A photograph shows whether a piece looks like a gold-coloured alloy rather than a grey one, and roughly how much tooth and ceramic is still attached. It cannot establish the alloy, so what is found at the bench is what counts.
Ready to ask about yours?
Send photographs and a person will tell you what the marks suggest, what looks plated, and whether anything ought to be seen by a specialist first. Free, in writing, no obligation.
Nothing is sold until you say yes. Decline and it all comes back by tracked post, free; ask for part of it back and a fresh offer is made for the rest.
If a piece is worth more than its metal
A signed brooch, a date-run sovereign, a watch that still runs, a piece of early silver: anything that looks worth more whole is flagged to you before it is treated as metal, and you can take it back at that point.