Dental amalgam is roughly half mercury by weight, mixed with a fine metallic powder containing silver, tin, copper, and smaller amounts of zinc and sometimes palladium.1PubMed Central. Determination and Calculations of Mercury Vapor Concentration and Energy Released from Freshly Condensed Dental Amalgams Having Various Copper Percentages within the Alloy That “about fifty percent mercury” figure surprises many people, but it is central to how amalgam works and why it has remained one of the most debated materials in dentistry for over a century. The exact recipe has shifted over the decades, and modern formulations differ from older ones in ways that matter for durability, corrosion, and the amount of mercury your filling releases.
What Goes Into the Powder
Before mercury enters the picture, the dentist or manufacturer starts with an alloy powder. Silver is the dominant metal in this powder, typically making up around two-thirds of it. Tin is the second-largest component, followed by copper, with small amounts of zinc rounding things out. A trace of palladium shows up in some modern products.2PubMed Central. Determination and Calculations of Mercury Vapor Concentration and Energy Released from Freshly Condensed Dental Amalgams Having Various Copper Percentages within the Alloy Each metal plays a role: silver provides strength, tin helps the alloy react with mercury at a reasonable pace, copper improves corrosion resistance, and zinc acts as a scavenger during manufacturing to prevent oxidation of the other metals.
A survey of 51 commercially available alloy powders, including both conventional and high-copper types, found that the conventional alloys showed little variation in their major components and generally met modern compositional standards. The high-copper alloys, as you would expect from the name, contained roughly four times the copper concentration of conventional products.3PubMed. A survey of the elemental composition of alloy for dental amalgam That difference in copper content turns out to be one of the most consequential changes in the history of filling materials.
How Mercury Turns Powder Into a Filling
Mercury is the ingredient that makes amalgam possible. When liquid mercury is mixed with the alloy powder, a chemical reaction called amalgamation begins. The mercury dissolves into the surfaces of the metal particles and forms new crystalline phases, binding the loose powder into a solid mass that can be packed into a prepared tooth cavity. Within minutes the mixture becomes workable but firm, and over the following hours it hardens to near-final strength.
The reaction does not consume all the mercury in a neat, tidy way. The original alloy particles remain partially intact, surrounded by newly formed matrix phases. Some of these phases are stronger and more corrosion-resistant than others, and the balance between them depends heavily on the alloy’s copper content. This is where the distinction between “conventional” and “high-copper” amalgam becomes important.
Why High-Copper Amalgam Replaced the Old Formulation
In conventional amalgam, one of the reaction products is a tin-mercury phase that dentists and researchers refer to as gamma-2. This phase is soft and prone to corrosion in the wet environment of the mouth. Its breakdown is the main reason older amalgam fillings tarnish, develop marginal gaps, and eventually fail. High-copper alloys were developed specifically to address this problem. When the copper content is raised, a copper-tin phase forms preferentially, and the troublesome tin-mercury phase is reduced or eliminated entirely.4PubMed. High copper alloys for dental amalgam
The practical result is a filling that resists corrosion better, creeps less under chewing forces, and lasts longer. High-copper amalgam has been the standard in most of the world since the late twentieth century, and conventional low-copper formulations are now rarely used. If you had a filling placed any time in the past few decades, it was almost certainly a high-copper product.
Particle Size and Setting Behavior
Beyond the chemical recipe, the physical form of the alloy powder matters. Amalgam powders come in lathe-cut (irregularly shaped) or spherical particles, and the particle size affects how the material handles in the dentist’s hands as well as its final properties. Research on dispersed palladium-containing amalgams showed that smaller dispersant particles led to faster setting, less expansion during hardening, higher compressive strength, and lower creep. Smaller particles also produced a more favorable corrosion profile and released less mercury vapor in the early stages after placement.5PubMed. Particle size effect on structure and properties of dispersed Pd-containing dental amalgam In other words, finer powder gives you a tighter, stronger, and more stable filling.
What Happens to Amalgam in Your Mouth
A filling sits in one of the harsher environments your body can offer: warm, wet, acidic at times, and constantly subjected to mechanical stress. Amalgam corrodes there, slowly. The corrosion products that form depend on the chemistry of your saliva, the pH shifts that happen when you eat, and whether the amalgam is in electrical contact with other metals (like a gold crown nearby).
Theoretical modeling of amalgam corrosion confirmed that all of the corrosion products reported in the dental literature can form, and it also identified two previously unreported possibilities: copper thiocyanate and silver thiocyanate. An important finding from the same work was that mercury-containing corrosion products cannot form on amalgam restorations that have no metallic contact with other materials.6PubMed. Release of elements due to electrochemical corrosion of dental amalgam That means a stand-alone amalgam filling, with no neighboring gold or other metal, is less likely to release mercury through corrosion than one sitting next to a different metallic restoration.
The acidity of the mouth also plays a role. Laboratory studies found that mercury dissolution from amalgam is relatively constant across a wide pH range, from mildly acidic to slightly alkaline. Only at extremely low pH, around 1, does the dissolution rate jump dramatically. A protective tin oxide film that forms on the surface of tin-containing phases accounts for much of the stability, and that film holds up well until the environment becomes severely acidic.7PubMed. Dissolution of mercury from dental amalgam at different pH values Normal saliva pH ranges from about 6 to 7, well within the stable zone.
Mercury Vapor Release From Fillings
The question most people really want answered about amalgam composition is whether the mercury in their fillings is making them sick. Mercury does escape from amalgam, primarily as vapor during chewing and brushing. But the quantities are small. An in vitro study that simulated chewing and brushing in an artificial mouth found that the steady-state mercury dose from a single amalgam filling was about 0.03 micrograms per day, far below the occupational threshold value of roughly 82 micrograms per day considered dangerous in the United States.8PubMed. Mercury release from dental amalgams: an in vitro study under controlled chewing and brushing in an artificial mouth
A study of patients who reported symptoms they attributed to their amalgam fillings provides additional context. The calculated daily uptake of inhaled mercury vapor from their restorations was less than five percent of the lower bound at which subtle symptoms have been seen in sensitive individuals, according to WHO reference values. The patients who blamed their fillings did not have higher mercury uptake or higher blood and urine mercury levels than a control group without symptoms.9PubMed. Mercury vapor release from dental amalgam in patients with symptoms allegedly caused by amalgam fillings That does not prove amalgam is harmless for every person on earth, but it does suggest that the mercury vapor from a typical set of fillings falls well below levels known to cause problems.
The main exposure window for mercury from amalgam is actually during placement or removal of the restoration. Once the amalgamation reaction is complete, the mercury is largely locked into stable crystalline phases and the ongoing release drops significantly.10PubMed Central. The dental amalgam toxicity fear: a myth or actuality This is why dentists use high-volume suction and water spray when drilling out old fillings.
Where Does the Released Mercury Go in the Body?
Even at low doses, some mercury from amalgam does get absorbed. A primate study in which monkeys received amalgam fillings totaling less than about one gram found measurable mercury deposits in multiple organs, including the kidneys, liver, lungs, spinal ganglia, anterior pituitary, and adrenal glands. The researchers concluded that mercury from amalgam is absorbed through the lungs and intestinal tract and eventually distributes to most organs, including the central nervous system.11Experimental and Molecular Pathology. Traces of mercury in organs from primates with amalgam fillings
This sounds alarming on its face, and it is the finding that anti-amalgam advocates cite most often. But detectable does not mean harmful. The body handles low-level mercury exposure through ongoing clearance by the kidneys and liver. The clinical question is whether the amounts deposited from dental fillings are large enough to cause functional harm, and that question has been tested directly.
The Children’s Amalgam Trial
One of the strongest pieces of evidence on amalgam safety is a randomized clinical trial that assigned children who needed fillings to receive either amalgam or mercury-free composite restorations, then followed them for five years. The primary endpoint was change in full-scale IQ. There was no statistically significant difference between the two groups: children in the amalgam group gained about three IQ points over five years, while those in the composite group gained about two, a gap that was not meaningful. Secondary measures of memory, visuomotor function, and kidney health also showed no significant differences.12PubMed. Neuropsychological and renal effects of dental amalgam in children: a randomized clinical trial The researchers noted that very small effects could not be entirely ruled out, but that the findings did not support making treatment decisions based on fears about amalgam’s health effects.
Local Reactions in the Mouth
While systemic toxicity from amalgam appears to be a non-issue for the vast majority of people, local tissue reactions do occur in a small number of patients. The most well-documented is the oral lichenoid lesion, a white, sometimes painful patch on the gum or cheek tissue next to an amalgam filling. This represents a delayed immune response, specifically a type IV cell-mediated hypersensitivity reaction, to mercury or another component of the amalgam.13PubMed Central. Lichenoid reaction associated with silver amalgam restoration in a Bombay blood group patient: A case report
A prospective study in Spain found lichenoid lesions near amalgam restorations in a small number of patients, and in all cases the lesions were adjacent to or in direct contact with the fillings. The amalgam restorations involved were typically old, averaging about 27 years in the mouth, with most showing rough, blackish surfaces. When the amalgam fillings were replaced with composite, about 70 percent of patients showed improvement within three months, and one patient’s lesions disappeared entirely by six months.14PubMed Central. Oral lichenoid lesions associated with amalgam restorations: A prospective pilot study addressing the adult population of the Basque Country If you notice a persistent white patch on the tissue right next to an old silver filling, it is worth asking your dentist about it.
Occupational Risk for Dental Workers
People who work with amalgam every day face a different exposure profile than patients. A review of mercury exposure among dental personnel across several countries found that dental workers consistently had higher mercury levels in blood, urine, and other biological samples compared to control groups. Some workplace practices, like hand-mixing amalgam or poor ventilation, increased exposure. However, even in these studies, exposure levels generally remained below recommended occupational guidelines.15PubMed Central. A Review of Mercury Exposure and Health of Dental Personnel Modern dental offices use pre-capsulated amalgam, which eliminates the old practice of manually mixing mercury with alloy powder and substantially reduces vapor exposure for the dental team.
Amalgam Versus Composite for Longevity
The composition of amalgam gives it mechanical properties that are hard to match. It is strong under compression, self-sealing as it corrodes slightly at the margins, and forgiving of less-than-perfect technique. Composite resin fillings, which are tooth-colored and mercury-free, have improved dramatically over the past two decades but still have a shorter track record for durability in the back teeth where chewing forces are highest.
A systematic review of long-term studies found that amalgam restorations had median survival times exceeding 16 years, compared to about 11 years for composite in posterior teeth.16PubMed Central. Longevity of Amalgam Versus Composite Resin Restorations in Permanent Posterior Teeth: A Systematic Review Another review concluded that while correctly placed composite fillings can sometimes match amalgam’s lifespan, overall amalgam outlasts composite in both posterior and anterior teeth for most observation periods beyond three to five years.17PubMed Central. Longevity of dental amalgam in comparison to composite materials For large cavities in molars, especially in patients who grind their teeth or have high caries risk, this longevity advantage is real and clinically meaningful.
Environmental Concerns and the Minamata Convention
The mercury in dental amalgam does not just affect patients and dental workers. It enters the environment through wastewater from dental offices, cremation of bodies with amalgam fillings, and improper disposal of amalgam scrap. Dental offices are equipped with amalgam separators to capture particles before they reach the sewer system, but the performance of these devices varies widely. Testing of three different separator units showed mercury removal efficiency ranging from about 27 percent to 95 percent depending on the size of the clinic and the unit used.18PubMed. Particle versus mercury removal efficiency of amalgam separators Even clinics with separators can emit substantial mercury in their wastewater if the discharge system is poorly designed or infrequently maintained.19PubMed. High mercury emissions from dental clinics despite amalgam separators
These environmental concerns were a driving force behind the Minamata Convention on Mercury, an international treaty that went into effect in 2017. The convention calls for a voluntary phase-down of dental amalgam use and commits countries to measures like promoting research into alternatives and restricting amalgam use in vulnerable populations. Critics have pointed out that the convention falls short by not requiring binding or measurable targets for phasing out amalgam.20Science of The Total Environment. The Minamata Convention on Mercury: Attempting to address the global controversy of dental amalgam use and mercury waste disposal Several European countries have gone further on their own, with Norway, Sweden, and Denmark effectively banning amalgam for routine use.
Mercury-Free Metal Alternatives
The push to eliminate mercury has led researchers to explore metallic filling materials that mimic amalgam’s handling properties without using mercury at all. The most prominent candidate has been gallium-based alloys, which use liquid gallium in place of liquid mercury to bind a similar alloy powder.21PubMed. A 3-year clinical evaluation of a gallium restorative alloy One commercial product, marketed under the brand name Galloy, was tested in clinical trials comparing it against a high-copper mercury-based amalgam.22British Dental Journal. A study into the performance of a gallium-based restorative material
The results were disappointing. Gallium alloys expanded excessively after placement, leading to cracked teeth in some cases, and their mechanical properties did not reliably match those of conventional high-copper amalgam.23PubMed. A comparison of the mechanical properties of a gallium-based alloy with a spherical high-copper amalgam Galloy was eventually withdrawn from the market. As a result, the practical alternatives to amalgam today remain composite resins, glass ionomer cements, and ceramic inlays rather than another metal amalgam. None of them are a perfect drop-in replacement for every situation where amalgam excels, which is why amalgam, despite its controversial mercury content, continues to be used in many parts of the world.
Recycling Amalgam Scrap
When old amalgam fillings are drilled out or when excess material is left over after a procedure, the scrap is not simply thrown away. It retains valuable metals, primarily silver and mercury. Dental offices are expected to collect amalgam waste in sealed containers for proper handling. Recovery processes typically involve vacuum distillation to separate the mercury from the solid metals: heating the scrap causes the mercury to vaporize and recondense in a collection vessel, while the remaining alloy is processed to recover silver. Laboratory work using 150 grams of dental amalgam scrap demonstrated that 50 to 80 percent of the silver could be recovered using a chemical reduction process, yielding silver of about 70 to 80 percent purity. The total time required ranged from about five to ten hours depending on conditions. Both the recovered mercury and silver have commercial value, giving dental offices a financial incentive to handle amalgam waste responsibly rather than letting it go down the drain.