Is Orpiment Toxic?
Orpiment, As2S3, is 60.91% arsenic by mass. COSHH 2002 lists arsenic in Schedule 1; HSE EH40 lists arsenic compounds at 0.1 mg per cubic metre. Not advice.
Orpiment is arsenic sulfide, As2S3, and 60.91% arsenic by mass, hardness 1.5 to 2. UK regulation names arsenic, not orpiment: COSHH 2002 lists arsenic in Schedule 1 for its definition of carcinogen, and HSE EH40 sets arsenic and arsenic compounds except arsine, as As, at 0.1 mg per cubic metre over 8 hours, notation Carc. This is not health advice.
In short
- What it is. The Handbook of Mineralogy orpiment sheet gives As2S3, stated to be very near that formula. Worked on standard atomic weights, arsenic is 149.84 of a formula mass of 246.02: 60.91% by mass. The IMA-CNMNC Master List lists orpiment As2S3, grandfathered.
- What UK regulation lists. COSHH 2002, Schedule 1 names Arsenic among the other substances and processes to which the definition of carcinogen relates. HSE EH40/2005 Workplace exposure limits lists arsenic and arsenic compounds except arsine (as As) at a long-term limit of 0.1 mg per cubic metre, 8-hour, with no short-term figure, notation Carc: capable of causing cancer and/or heritable genetic damage.
- What EH40 does not do. It does not name orpiment. Its list of synonyms has entries for barite and calcite but none for orpiment, realgar or stibnite. EH40 also says that the absence of a substance from its list does not indicate that it is safe. This page does not decide how either entry applies to a specimen.
- The association-line lookup. The orpiment sheet lists six associates: stibnite, realgar, arsenic, calcite, barite and gypsum. Read against EH40, realgar and native arsenic fall under the arsenic entry, as orpiment does; stibnite under antimony (as Sb); and calcite, barite and gypsum under named dust entries. The table below sets each out, quoted rather than interpreted.
- Not advice. This is not health, safety or legal advice. EH40 and COSHH 2002 govern workplaces; a concern about exposure belongs with a clinician. The general site note is toxic minerals in a collection.
| Mineral | Formula (IMA) | EH40 entry it falls under | How EH40 reaches it | Element by mass, worked | Long-term limit, 8-hr TWA, as printed | Notation |
|---|---|---|---|---|---|---|
| Orpiment | As2S3 | Arsenic and arsenic compounds except arsine (as As) | Not named; arsenic compound | 60.91% As | 0.1 mg per cubic metre | Carc |
| Realgar | AsS | Arsenic and arsenic compounds except arsine (as As) | Not named; arsenic compound | 70.03% As | 0.1 mg per cubic metre | Carc |
| Arsenic (native) | As | Arsenic and arsenic compounds except arsine (as As) | The element is in the entry name | 100% As | 0.1 mg per cubic metre | Carc |
| Stibnite | Sb2S3 | Antimony and compounds except stibine (as Sb) | Not named; antimony compound | 71.69% Sb | 0.5 mg per cubic metre | None printed |
| Baryte (barite) | Ba(SO4) | Barium sulphate | Synonym list: Barite, Barytes | - | Inhalable dust 10, respirable 4 mg per cubic metre | None printed |
| Calcite | Ca(CO3) | Marble | Synonym list: Calcite | - | Total inhalable 10, respirable 4 mg per cubic metre | None printed |
| Gypsum | Ca(SO4)(H2O)2 | Gypsum | Entry name; synonym list: Calcium sulphate | - | Inhalable dust 10, respirable 4 mg per cubic metre | None printed |
Is orpiment toxic? What it is made of, worked
Orpiment is arsenic sulfide, As2S3, 60.91% arsenic by mass; the sheet describes it as soft, sectile and often foliated, fibrous, granular or powdery. Method: with As 74.9216 and S 32.06, As2S3 has a formula mass of 246.02, of which arsenic is 149.84. The Handbook of Mineralogy orpiment sheet prints no analysis, only stated to be very near As2S3, so the ideal figure is the one available. The same arithmetic gives realgar 70.03%, the figure on is realgar safe to handle, and arsenopyrite 46.01%, the figure on is arsenopyrite dangerous.
What the sheet does not say. The Handbook is a descriptive reference. A whole-sheet search finds no use of the word toxic on the orpiment sheet; among the arsenic minerals fetched for this page, only the arsenolite sheet uses it. The orpiment sheet prints hardness 1.5 to 2, cleavage lamellae that are flexible, and a habit that includes powdery material; it gives no solubility line, and this page adds none.
What this page does not cover. Heating, grinding, ingestion, pigment use and dose are outside it, because none of the sources fetched here addresses orpiment in those terms. The species facts are on orpiment.
What UK regulation says about arsenic, quoted
Two documents name arsenic and neither names orpiment. COSHH 2002, Schedule 1 is headed other substances and processes to which the definition of carcinogen relates, and its list includes Arsenic. COSHH 2002, regulation 2 defines a carcinogen to include a substance referred to in Schedule 1, and defines a substance hazardous to health to include one for which the Health and Safety Executive has approved a workplace exposure limit.
The EH40 entry. HSE EH40/2005 Workplace exposure limits, fourth edition 2020, lists arsenic and arsenic compounds except arsine (as As) with a long-term exposure limit of 0.1 mg per cubic metre over an 8-hour time-weighted average reference period, no short-term figure, and the notation Carc, which it explains as capable of causing cancer and/or heritable genetic damage. Arsine has its own entry. The as As means the limit is expressed as arsenic; this page gives the arsenic mass fraction of each mineral and does not convert the limit into a mass of orpiment.
How the document describes itself. EH40 says COSHH imposes requirements by reference to its Table 1, which is therefore legally binding where it applies to work activities, and that the remainder is guidance. It also says that the absence of a substance from the list of limits does not indicate that it is safe. These are workplace documents; This is not health, safety or legal advice. Workplace questions belong with HSE; questions about exposure belong with a clinician.
The association-line lookup: orpiment’s neighbours in EH40
The association-line lookup: read each associate on the orpiment sheet against EH40’s list of synonyms first, and its group entries second. The sheet lists stibnite, realgar, arsenic, calcite, barite and gypsum. EH40’s list of synonyms, which maps other names to the name a substance is listed by in Table 1, has entries for barite and barytes (to barium sulphate), calcite (to marble) and calcium sulphate (to gypsum). It has no entry for orpiment, realgar or stibnite, which therefore fall to the group entries for arsenic compounds and for antimony and compounds except stibine (as Sb). Native arsenic is the element named in the arsenic entry.
Documented failure: what the lookup cannot say. The lookup shows which entry a word leads to; it does not show that a natural specimen is within the scope of that entry, and EH40 does not say. Gypsum, calcite and baryte reach dust entries, which apply to dust in air, not to a crystal. The antimony line is there because stibnite is the first associate, and stibnite carries 71.69% antimony, the figure on its own page. Barium is covered on is baryte toxic and is not repeated.
The same structure on other pages. is cinnabar toxic quotes EH40’s mercury entry for cinnabar in the same way; the general note is toxic minerals in a collection. Neither this page nor those turns a workplace limit into a statement about a specimen in a cabinet.
Does orpiment break down? Light, powder and the realgar link
The orpiment sheet prints no light statement; the realgar sheet does, and names orpiment as one of realgar’s products. A script search of the orpiment sheet for light, exposure and disintegrat finds nothing outside its optical data. The realgar sheet reads: disintegrates on long exposure to light to a powder composed of pararealgar or arsenolite and orpiment. So a yellow crust on old realgar may be orpiment, pararealgar or arsenolite; only testing can say which.
The conservation source. The NatSCA leaflet on the care and conservation of geological specimens says light will irreversibly damage some mineral species and that the arsenic sulphide group of minerals are the most susceptible, including realgar; it recommends that susceptible species be stored in light-proof containers. It does not name orpiment. Its storage line is the leaflet’s, quoted; this page offers no handling or storage recipe of its own. See which minerals fade in light.
Where to go. HSE for workplace duties under COSHH 2002 and EH40; a clinician for any question about exposure. This hazard page is indexed with the others at the collecting guides. Nothing on this site is offered for sale; the wanted list is the only commercial route.