Tof-SIMS spectra of historical inorganic pigments: Copper-, zinc-, arsenic-, and phosphorus-containing pigments in negative polarity

ToF-SIMS imaging can detect a wide range of elements, so colored minerals can be identified and mapped, given that reference spectra are available for reliable assignment. This paper provides a comparative ToF-SIMS spectral database for eight historical inorganic pigments, containing copper carbonate, zinc oxide or phosphates, vanadium oxide, calcium phosphates, or arsenic sulfide. Numerous possibilities for mass interferences exist between combinations of these elements. Without being able to refer to comparative datasets that provide an overview of the fragment ions for each pigment, it would be difficult to accurately identify one of them as an unknown pigment. Although not presented in the paper, titanium oxide, used as a pigment from the 1920s, may exhibit additional interferences to be considered as TiO⁻ has an m/z of 62.94.

The reference materials were supplied by specialized manufacturers selling pigments reproducing historical recipes as closely as possible, such as Kremer Pigmente GmbH. Pigments were available as finely ground powder of variable particle size (between one and a few tens of μm) conditioned in glass sealed vials. In paintings, pigment particles are dispersed in the binder and not dissolved, so all references were analyzed without any preparation treatment. Natural and synthetic pigments may contain impurities, and specimen compositions described below are the major components as found on the material data sheets provided by the suppliers.

A stainless-steel spatula was cleaned with propan-2-ol, and the powder was directly deposited on a 1 × 1 mm² piece of a conductive double-sided tape (3M Electrically Conductive Adhesive Transfer Tape 9703) already fixed to a 1 cm × 1 cm × 0.125 mm stainless steel plate (17-7PH, Goodfellow Cambridge Limited, UK) fitting the dimensions of the sample holder. A hand press covered with a clean aluminum foil was applied onto the powder to fix it to the tape. This allowed to operate in the analysis chamber under ultrahigh vacuum (10⁻⁸–10⁻⁹ hPa) while ensuring a flat surface. For highly toxic pigments, such as the arsenic sulfide Realgar (#01949), a pellet with a flat surface was prepared using a press (Mini-Pellet Press P/N GS03940, Specac Ltd, UK) to limit dust formation.

All spectra were obtained on an IONTOF TOF SIMS IV equipped with a 25 keV bismuth liquid metal ion gun and an argon gas cluster ion beam (GCIB). Before all analyses, a 500 × 500 μm² surface was cleaned using 1500–2000 Ar clusters with a kinetic energy of 20 keV (sputter ion dose of 2 × 10¹⁵ ions/cm²). Analyses were then performed on a 250 × 250 μm² area centered in this larger cleaned area, rastered over 128 × 128 pixels using the high current bunched mode. Primary ion dose (25 keV Bi₃⁺) was 4.35 × 10¹¹ ions/cm². Surface potential was compensated, and charging of the sample was compensated with the low-energetic 20 eV electrons of the flood gun.

Data were calibrated using sets of peaks as similar as possible for all spectra, using peaks that could be confidently identified. Copper and zinc both have distinctive isotopic patterns ensuring confident attribution of the peaks, so #01951, #01952, and #01953 data were calibrated using lists including Cu⁻ and Cu₃O₃⁻ ions, and #01954 data were calibrated using ZnO₂⁻, Zn₂O₃⁻, Zn₃O₄⁻, ⁶⁶Zn₂⁶⁸ZnZn₂O₆⁻, ⁶⁶Zn₂⁶⁸Zn₂Zn₄O₈⁻, and ⁶⁶Zn₂⁶⁸Zn₂Zn₅O₁₀⁻. When possible, data were calibrated using lists including main ions, namely, C⁻, C₂⁻, C₃⁻, C₈⁻, C₁₀⁻, and C₁₃⁻ for #01947, CaO⁻, SiO₂⁻, PO₂⁻, SiO₃⁻, and PO₃ for #01948, and S₂⁻, AsS₂⁻, As₂S₃⁻, and As₃S₄ for #01949. No bismuth- and vanadium-containing ions were used to calibrate the #01950 dataset since bismuth has no isotope and is close in mass to VP₂O₆. Hence, #01950 data were calibrated using hydrocarbon and chlorine ions CH⁻, CH₂⁻, C₂⁻, Cl⁻, C₃⁻, and C₄H⁻. These calibration lists allowed for a good coverage of the mass range of interest for each specimen, and they are fully detailed in the corresponding tables below. Pigments contain diverse metallic oxides, hydroxides, carbonates, and even hydrocarbons. They are found inside painting layers that are mostly organic materials. All these compounds do not follow the same ionization processes and have different ionization kinetics.¹ In this context, the choice for the calibration ions is a crucial step to ensure proper peak attribution. If the analytical question aims at identifying the nature of a pigment particle, then the calibration must include inorganic ions, such as the list above, spanning all the mass range of interest and if possible of the same ion family as those of interest. However, if the analytical question focuses mainly on the identification of organic materials, the calibration must include known low-mass hydrocarbon ions. The safest strategy when interpreting data from a painting cross section would be to rely on several calibration sets adapted to the ions of interest and that can be confidently defined independently of the sample's unknowns.

Figures and tables presented below contain the major peaks for negative polarity. Each pigment is uniquely identified thanks to an accession number, and they are grouped according to their composition and described in the following section. Their historical relevance is briefly described based on current literature.^2–6 The spectrum ID# refers to that number.

Specimen: Peach black (Kremer Pigmente Ref. 12010)

CAS Registry #: Unknown

Specimen Characteristics: Unknown homogeneity; solid; amorphous; dielectric; inorganic compound; powder

Chemical Name: Amorphous carbon

Source: Kremer Pigmente GmbH

Specimen Composition: Amorphous carbon (charred peach kernel)

Form: Ground pigment powder

History and Significance: Black pigments derived from charcoals are rarely detectable by the analytical techniques used to investigate pigments such as XRF, while carbon-based black pigments have been used in every society since the domestication of fire. Peach black, for instance, only requires the calcination of kernels. Identifying this pigment is of interest, as they are omnipresent in historical artistic practice. ToF-SIMS is able to map and identify micrometric particles of carbon-base pigments.

As Received Condition: Ground pigment powders were stored in a sealed glass vial in a wooden box, protected from daylight. Stored at room temperature.

Analyzed Region: Areas of 250 × 250 μm²

Ex Situ Preparation/Mounting: Powder was deposited and pressed onto a conductive double-sided tape attached to stainless steel 1 × 1 cm² plates. The resulting deposit of powder had a flat surface and was fixed in a suitable way for the vacuum chamber. The plate was directly mounted on a “backmount” sample holder (IONTOF).

In Situ Preparation: None

Charge Control Conditions and Procedures: A low-energy electron flood gun was used. Bias voltage between 20 and −30 V. Flood gun filament current of 2.35 A. Surface potential was corrected (sample dependent).

Temp. During Analysis: 300 K

Pressure During Analysis: Between 1 × 10⁻⁵ and 1 × 10⁻⁶ Pa

Pre-analysis Beam Exposure: Ar_1500−2000, 20 keV, 2 × 10¹⁵ ions/cm²

Specimen: Bone black (Kremer Pigmente Ref. 47100)

CAS Registry #: 8021-99-6

Specimen Characteristics: Unknown homogeneity; solid; amorphous; dielectric; inorganic compound; powder

Chemical Name: Bone charcoal

Source: Kremer Pigmente GmbH

Specimen Composition: Amorphous carbon and calcium phosphates

Form: Ground pigment powder

History and Significance: Bone black is made by carbonization of animal bones at temperatures over 400 °C but not above 800 °C. It mainly consists of calcium phosphate and carbonized organics. It is very frequently used in paintings, especially in preparation layers.

As Received Condition: Ground pigment powders were stored in a sealed glass vial in a wooden box, protected from daylight. Stored at room temperature.

Analyzed Region: Areas of 250 × 250 μm²

Ex Situ Preparation/Mounting: Powder was deposited and pressed onto a conductive double-sided tape attached to stainless steel 1 × 1 cm² plates. The resulting deposit of powder had a flat surface and was fixed in a suitable way for the vacuum chamber. The plate was directly mounted on a “backmount” sample holder (IONTOF).

In Situ Preparation: None

Charge Control Conditions and Procedures: A low-energy electron flood gun was used. Bias voltage between 20 and −30 V. Flood gun filament current of 2.35 A. Surface potential was corrected (sample dependent).

Temp. During Analysis: 300 K

Pressure During Analysis: Between 1 × 10⁻⁵ and 1 × 10⁻⁶ Pa

Pre-analysis Beam Exposure: Ar_1500−2000, 20 keV, 2 × 10¹⁵ ions/cm²

Specimen: Realgar/red orpiment (Kremer Pigmente Ref. 10800)

CAS Registry #: 1303-33-9

Specimen Characteristics: Unknown homogeneity; solid; unknown crystallinity; dielectric; inorganic compound; powder

Chemical Name: Arsenic (III) sulfide (natural mineral)

Source: Kremer Pigmente GmbH

Specimen Composition: Natural arsenic sulfide α-As₄S₄

Form: Ground pigment powder

History and Significance: A mineral prized for its bright red color that was used until the beginning of the 19th century. Found in volcanic and geothermal regions, its geographic origin varies. It was found on luxurious articles in antiquity to show opulence and was used as ritual offerings and in cosmetics, despite its high toxicity. It exists in rare natural and synthetic forms. Its occurrence is difficult to investigate due to light instability, converting it to yellow pararealgar (As₄S₄), thereby often mistaken for other yellow As pigments unless crystalline phases can be differentiated by the analytical techniques used. Red realgar can be found in environments where the absence of light preserves it.

As Received Condition: Ground pigment powders were stored in a sealed glass vial in a wooden box, protected from daylight. Stored at room temperature.

Analyzed Region: Areas of 250 × 250 μm²

Ex Situ Preparation/Mounting: Because of the toxicity of arsenic sulfide, a pellet was prepared using a Specac Mini Pellet Press. A flat surface was obtained, and dust formation was limited when handling the sample. The pellet was directly mounted on a “backmount” sample holder (IONTOF).

In Situ Preparation: None

Charge Control Conditions and Procedures: A low-energy electron flood gun was used. Bias voltage between 20 and −30 V. Flood gun filament current of 2.35 A. Surface potential was corrected (sample dependent).

Temp. During Analysis: 300 K

Pressure During Analysis: Between 1 × 10⁻⁵ and 1 × 10⁻⁶ Pa

Pre-analysis Beam Exposure: Ar_1500−2000, 20 keV, 2 × 10¹⁵ ions/cm²

Specimen: Bristol yellow medium (Kremer Pigmente Ref. 43111)

CAS Registry #: 7779-90-0 and 14059-33-7

Specimen Characteristics: Inhomogeneous; solid; unknown crystallinity; dielectric; inorganic compound; powder

Chemical Name: Trizinc bis(ortho)phosphate + bismuth vanadium tetraoxide

Source: Kremer Pigmente GmbH

Specimen Composition: Zn₃(PO₄)₂ + BiVO₄

Form: Ground pigment powder

History and Significance: Due to their toxicity, lead pigments have been gradually prohibited over the past two centuries. Alternative yellow pigments such as modified bismuth yellows (Bristol yellows) were produced to replace them. Bristol yellows have been used since the 20th century to imitate Naples yellow as they have similar optical properties. Copper and zinc compounds can have mass interference with Bristol yellow ions, so their distinction is of interest.

As Received Condition: Ground pigment powders were stored in a sealed glass vial in a wooden box, protected from daylight. Stored at room temperature.

Analyzed Region: Areas of 250 × 250 μm²

Ex Situ Preparation/Mounting: Powder was deposited and pressed onto a conductive double-sided tape attached to stainless steel 1 × 1 cm² plates. The resulting deposit of powder had a flat surface and was fixed in a suitable way for the vacuum chamber. The plate was directly mounted on a “backmount” sample holder (IONTOF).

In Situ Preparation: None

Charge Control Conditions and Procedures: A low-energy electron flood gun was used. Bias voltage between 20 and −30 V. Flood gun filament current of 2.35 A. Surface potential was corrected (sample dependent).

Temp. During Analysis: 300 K

Pressure During Analysis: Between 1 × 10⁻⁵ and 1 × 10⁻⁶ Pa

Pre-analysis Beam Exposure: Ar_1500−2000, 20 keV, 2 × 10¹⁵ ions/cm²

Specimen: Azurite (Kremer Pigmente Ref. 10207)

CAS Registry #: 12069-69-1

Specimen Characteristics: Inhomogeneous; solid; unknown crystallinity; dielectric; inorganic compound; powder

Chemical Name: Basic copper (II) carbonate

Source: Kremer Pigmente GmbH

Specimen Composition: Cu₃(CO₃)₂(OH)₂

Form: Ground pigment powder

History and Significance: A natural blue to turquoise-blue-green copper carbonate that results from the erosion of copper deposits. It was used in many contexts over the centuries and was considered a fairly precious material.

As Received Condition: Ground pigment powders were stored in a sealed glass vial in a wooden box, protected from daylight. Stored at room temperature.

Analyzed Region: Areas of 250 × 250 μm²

Ex Situ Preparation/Mounting: Powder was deposited and pressed onto a conductive double-sided tape attached to stainless steel 1 × 1 cm² plates. The resulting deposit of powder had a flat surface and was fixed in a suitable way for the vacuum chamber. The plate was directly mounted on a “backmount” sample holder (IONTOF).

In Situ Preparation: None

Charge Control Conditions and Procedures: A low-energy electron flood gun was used. Bias voltage between 20 and −30 V. Flood gun filament current of 2.35 A. Surface potential was corrected (sample dependent).

Temp. During Analysis: 300 K

Pressure During Analysis: Between 1 × 10⁻⁵ and 1 × 10⁻⁶ Pa

Pre-analysis Beam Exposure: Ar_1500−2000, 20 keV, 2 × 10¹⁵ ions/cm²

Specimen: Malachite, natural (Kremer Pigmente Ref. 10300)

CAS Registry #: 1319-53-5

Specimen Characteristics: Inhomogeneous; solid; unknown crystallinity; dielectric; inorganic compound; powder

Chemical Name: Basic cupric carbonate

Source: Kremer Pigmente GmbH

Specimen Composition: CuCO₃⋅Cu(OH)₂

Form: Ground pigment powder

History and Significance: A natural green to turquoise-green copper carbonate, which results from the erosion of copper deposits. It is one of the earliest known bright green pigments, and a dominant one until the mid-18th century. It was considered a fairly precious material. It consists of rather large spherical particles.

As Received Condition: Ground pigment powders were stored in a sealed glass vial in a wooden box, protected from daylight. Stored at room temperature.

Analyzed Region: Areas of 250 × 250 μm²

Ex Situ Preparation/Mounting: Powder was deposited and pressed onto a conductive double-sided tape attached to stainless steel 1 × 1 cm² plates. The resulting deposit of powder had a flat surface and was fixed in a suitable way for the vacuum chamber. The plate was directly mounted on a “backmount” sample holder (IONTOF).

In Situ Preparation: None

Charge Control Conditions and Procedures: A low-energy electron flood gun was used. Bias voltage between 20 and −30 V. Flood gun filament current of 2.35 A. Surface potential was corrected (sample dependent).

Temp. During Analysis: 300 K

Pressure During Analysis: Between 1 × 10⁻⁵ and 1 × 10⁻⁶ Pa

Pre-analysis Beam Exposure: Ar_1500−2000, 20 keV, 2 × 10¹⁵ ions/cm²

Specimen: Verdigris, synthetic (Kremer Pigmente Ref. 44450)

CAS Registry #: 6046-93-1

Specimen Characteristics: Inhomogeneous; solid; unknown crystallinity; dielectric; inorganic compound; powder

Chemical Name: Copper-(II)-acetate-1-hydrate

Source: Kremer Pigmente GmbH

Specimen Composition: Cu(CH₃COO)₂⋅[Cu(OH)₂]₃⋅2H₂O

Form: Ground pigment powder

History and Significance: A family of synthetic pigments, containing variations in copper acetate, with green to blue-green hues. Synthesized since antiquity, it is an acetate copper salt produced by the reaction of acetic acid with copper and has been particularly common in wine-growing regions. It has a characteristic vinegar odor. A better knowledge of the ion signal of Verdigris pigments is of interest. Indeed, considered unstable, it reacts with binders and other pigments forming soaps and other copper salts, respectively, making its identification in a painting often ambiguous. Degradation products result in a brown tint.

As Received Condition: Ground pigment powders were stored in a sealed glass vial in a wooden box, protected from daylight. Stored at room temperature.

Analyzed Region: Areas of 250 × 250 μm²

Ex Situ Preparation/Mounting: Powder was deposited and pressed onto a conductive double-sided tape attached to stainless steel 1 × 1 cm² plates. The resulting deposit of powder had a flat surface and was fixed in a suitable way for the vacuum chamber. The plate was directly mounted on a “backmount” sample holder (IONTOF).

In Situ Preparation: None

Charge Control Conditions and Procedures: A low-energy electron flood gun was used. Bias voltage between 20 and −30 V. Flood gun filament current of 2.35 A. Surface potential was corrected (sample dependent).

Temp. During Analysis: 300 K

Pressure During Analysis: Between 1 × 10⁻⁵ and 1 × 10⁻⁶ Pa

Pre-analysis Beam Exposure: Ar_1500−2000, 20 keV, 2 × 10¹⁵ ions/cm²

Specimen: Zinc white (Kremer Pigmente Ref. 46300)

CAS Registry #: 1314-13-2

Specimen Characteristics: Inhomogeneous; solid; unknown crystallinity; dielectric; inorganic compound; powder

Chemical Name: Zinc oxide

Source: Kremer Pigmente GmbH

Specimen Composition: ZnO

Form: Ground pigment powder

History and Significance: A widely used substitute for lead-containing whites that was first produced in the 19th century by Winsor & Newton (1834) with Michael Faraday.

As Received Condition: Ground pigment powders were stored in a sealed glass vial in a wooden box, protected from daylight. Stored at room temperature.

Analyzed Region: Areas of 250 × 250 μm²

Ex Situ Preparation/Mounting: Powder was deposited and pressed onto a conductive double-sided tape attached to stainless steel 1 × 1 cm² plates. The resulting deposit of powder had a flat surface and was fixed in a suitable way for the vacuum chamber. The plate was directly mounted on a “backmount” sample holder (IONTOF).

In Situ Preparation: None

Charge Control Conditions and Procedures: A low-energy electron flood gun was used. Bias voltage between 20 and −30 V. Flood gun filament current of 2.35 A. Surface potential was corrected (sample dependent).

Temp. During Analysis: 300 K

Pressure During Analysis: Between 1 × 10⁻⁵ and 1 × 10⁻⁶ Pa

Pre-analysis Beam Exposure: Ar_1500−2000, 20 keV, 2 × 10¹⁵ ions/cm²

Manufacturer and Model: IONTOF, TOF SIMS IV

Analyzer Type: Time-of-flight

Sample Rotation: No

Rotation Rate: 0 rpm

Oxygen Flood Source: None

Oxygen Flood Pressure: N/A

Other Flood Source: None

Other Flood Pressure: N/A

Unique Instrument Features Used: None

Energy Acceptance Window: 20 eV

Post-acceleration Voltage: 10 000 eV

Sample Bias: 10–20 eV

Specimen Normal-to-analyzer (Θe): 0°

Purpose of this Ion Source: Analysis beam

Ion Source Manufacturer: IONTOF GmbH

Ion Source Model: Liquid metal ion gun (LMIG) with bismuth cluster (25 keV)

Beam Mass Filter: Yes

Beam Species and Charge State: Bi₃⁺

Beam Gating Used: Double pulsed + Bunched

Additional Beam Comments: None

Beam Voltage: 25 000 eV

Net Beam Voltage (impact voltage): 25 000 eV

Ion Pulse Width: 0.8–1.2 ns

Ion Pulse Rate: 5–10 kHz

DC Beam Current: ∼10 nA

Pulsed Beam Current: ∼0.0004 nA

Current Measurement Method: Faraday cup

Beam Diameter: ∼2 μm

Beam Raster Size: 250 × 250 μm²

Raster Pixel Dimensions: 128 × 128

Beam Incident Angle: 45°

Source-to-Analyzer Angle: 45°

Purpose of this Ion Source: Sputtering beam

Ion Source Manufacturer: IONTOF GmbH

Ion Source Model: Argon gas cluster ion beam (GCIB)

Beam Mass Filter: Yes

Beam Species and Charge State: Ar_n⁺ with n = 1500–2000

Beam Gating Used: Wien filter

Additional Beam Comments: Used as sputter gun, not analysis mode.

Beam Voltage: 20 000 eV

Net Beam Voltage (impact voltage): 20 000 eV

Ion Pulse Width: N/A

Ion Pulse Rate: N/A

DC Beam Current: ∼10 nA

Pulsed Beam Current: N/A

Current Measurement Method: Faraday cup

Beam Diameter: ∼50 μm

Beam Raster Size: 500 × 500 μm²

Raster Pixel Dimensions: N/A

Beam Incident Angle: 45°

Source-to-Analyzer Angle: 45°