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General information for laboratory and clinic staff, not medical advice and not a procedure guide.
Short answer
Bismuth trioxide (also called bismuth oxide, formula Bi2O3) is a heavy-metal oxide powder that dental and endodontic cements use as a radiopacifier: it makes the set material show up on an X-ray image. A 2020 laboratory study found it is not chemically inert, because it reacts even in water and saline (with CO2 present) to form bismuth subcarbonate and can darken in some solutions.
Definition and scope
A radiopacifier is an additive that makes a material visible on radiographs; the general concept is explained in our guide to what radiopaque means. Camilleri and colleagues (PLoS One, 2020) note that agents commonly used for this purpose include barium sulfate, calcium tungstate, and zirconium, tantalum and bismuth oxides, and that radiopacifiers are expected to be inert: neither reacting with the host material nor with its environment.
This page covers bismuth trioxide as a material ingredient in dental and endodontic products. It does not cover clinical technique, and it does not rate individual products. For what counts as a device in the first place, see what is a medical device.
How it is used in practice
In a cement, the oxide is mixed in as a powder by weight so that the set material appears on a radiograph. Marciano and colleagues (Brazilian Oral Research, 2013) tested calcium silicate cement (CSC) mixed with 0%, 15%, 20%, 30% and 50% bismuth oxide by weight, with mineral trioxide aggregate (MTA) as the control. Radiopacity was tested according to ISO 6876/2001, which the article describes as setting a minimum of 3 mm of aluminium equivalent. Imaging basics are covered in what is an X-ray machine.
Bismuth oxide, bismuth subcarbonate and related forms
Bismuth subcarbonate is a reaction product of the oxide rather than a separate additive. Camilleri and colleagues immersed 10 g of Bi2O3 in about 40 mL of each test solution, in light or dark, with and without CO2, for 1, 4, 12 and 24 weeks.
What formed in each solution (Camilleri et al., 2020)
| Solution | What the study reported |
|---|---|
| Deionized water (control) | Bismuth subcarbonate, (BiO)4CO3(OH)2, formed in the presence of CO2 |
| 0.9% sodium chloride | Bismuth subcarbonate formed in the presence of CO2 |
| 1 mol/L sodium hydroxide | Bismuth subcarbonate formed in the presence of CO2 |
| 10% sodium hypochlorite (pH about 12.4) | Sodium bismuthate formed rapidly; very dark brown to black |
| 1 mol/L hydrochloric acid | Bismuth oxychloride (BiOCl) formed; pale, almost white powder |
The authors also report that discolouration intensified with light over time, even though X-ray diffraction and infrared analysis showed no chemical difference between light and dark samples, and they note that the subcarbonate is light-sensitive.
Where it matters for labs, clinics and supply decisions
Two findings in the sources bear on how product documents are read. First, radiopacity depends on how much oxide a cement contains. In Marciano and colleagues’ tests, plain CSC did not reach the 3 mm aluminium minimum, every mix containing bismuth oxide did, and at least 15% was needed. MTA was more radiopaque than CSC with 0%, 15% and 20%, similar to CSC with 30%, and the 50% mix was more radiopaque than every other group.
Radiopacity by composition (Marciano et al., 2013)
| Mix | Result against the ISO 6876/2001 3 mm Al minimum |
|---|---|
| CSC, 0% bismuth oxide | Below the minimum |
| CSC, 15% and 20% | Above the minimum; MTA significantly more radiopaque |
| CSC, 30% | Above the minimum; statistically similar to MTA |
| CSC, 50% | Above the minimum; more radiopaque than every other group |
Second, the authors of the 2020 study argue that bismuth oxide should not be used in dental materials because of the reactions above, and point out that cements relying on zirconium dioxide or tantalum oxide are on the market instead. That is the authors’ position; the two sets of findings show that radiopacity and chemical behaviour are separate questions. Staff comparing products can read each manufacturer’s instructions for use and safety data sheet to see which radiopacifier is named. For general lab supply planning, see the lab safety equipment checklist.
Common misunderstandings
Is bismuth trioxide inert?
Not according to the 2020 study: it reacted in water and saline as well as in stronger solutions, which the authors say contradicts the common belief that the oxide is inert.
Does more bismuth oxide mean more discolouration?
Not in the 2013 cement study. At 30 days the plain CSC group had higher colour change values (ΔE) than the 30% and 50% bismuth oxide groups, and the authors concluded that radiopacity from bismuth oxide has no relation to colour change, suggesting other factors such as dentin interaction or light. That study measured only radiopacity and colour, so it says nothing about setting, mechanical or biological properties.
FAQ
What is bismuth trioxide used for in dental materials?
It is added as a radiopacifier so that a set cement is visible on radiographs.
What is the difference between bismuth trioxide and bismuth subcarbonate?
Bismuth subcarbonate, (BiO)4CO3(OH)2, is what the 2020 study found forming when the oxide sat in water, saline or sodium hydroxide with CO2 present.
What reacts with bismuth trioxide to turn it dark?
In the 2020 study, 10% sodium hypochlorite produced sodium bismuthate, very dark brown to black. Light also deepened discolouration over time.
Sources
- Camilleri J, Borg J, Damidot D, Salvadori E, Pilecki P, Zaslansky P, Darvell BW. PLoS One 2020;15(11):e0240634. https://pmc.ncbi.nlm.nih.gov/articles/PMC7657490 Checked 2026-10-30.
- Marciano et al. Analysis of the color alteration and radiopacity promoted by bismuth oxide in calcium silicate cement. Brazilian Oral Research 2013;27(4) (Jul-Aug 2013). https://www.scielo.br/j/bor/a/33DCj8MnvNY6jSnzGmV3RXL/?lang=en Checked 2026-10-30.
