ToF-SIMS spectra of historical inorganic pigments: Lead-based pigments in positive polarity Open Access

• Accession#: 01906, 01907, 01908, 01909, 01910, 01911, 01912, and 01913

• Technique: SIMS

• Specimen: Lead tin yellow I, Naples yellow, Lead white, Flake white 10μ, Flake white, Cerussite, Minium, and Litharge

• Instrument: IONTOF, TOF SIMS IV

• Major Species in Spectra: O, H, Si, S, Al, C, Na, Pb, K, Sb, and Sn

• Minor Species in Spectra: Hydrocarbon contamination

• Published Spectra: 8

• Spectral Category: Reference

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. Here, we provide a ToF-SIMS spectral database for eight lead-based inorganic pigments. Lead-based pigments have been commonly used in all painting techniques until the late 19th century and, therefore, are very likely to be found when analyzing a historical painting sample. In particular, the pigment generally referred to as “lead white” can have various compositions depending on the manufacturing process, and this paper compares a selection of them (namely #01908, #01909, #01910, and #01911). For each, a list of their fragmentation products is given.

The reference materials were supplied by specialized manufacturers selling pigments reproducing historical recipes as closely as possible, such as Kremer Pigmente GmbH, Masters Pigments, Laverdure, and Sennelier. 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 not solubilized, 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 conductive double-sided tape (3 M 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 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 to 10⁻⁹ hPa) while ensuring a flat surface.

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. The primary ion dose (25 keV Bi₃⁺) was 4.35 × 10¹¹ ions/cm². Surface potential was compensated.

All data were calibrated using the same set of peaks appearing in all spectra, namely, Na⁺ at m/z 22.99, Pb⁺ at m/z 207.98, Pb₂⁺ at m/z 415.95, Pb₂O⁺ at m/z 431.91, ²⁰⁶PbPb₂O₂⁺ at m/z 653.92, ²⁰⁶PbPb₃O₃⁺ at m/z 877.89, and ²⁰⁶PbPb₄O₄⁺ at m/z 1101.86. This calibration list allowed for a complete coverage of the mass range of interest, and the lead-containing calibration ions could easily be verified using their isotopic pattern. 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 (Ref. 1). In this context, the choice of 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 (such as C₂H₃⁺, C₃H₅⁺, C₄H₇⁺, and C₇H₇⁺). 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 positive 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 (Refs. 2–6). The spectrum ID# refers to that number.

Specimen: Lead Tin Yellow I (Kremer Pigmente Ref. 10100)

CAS Registry #: 12143-43-0

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

Chemical Name: Lead stannate

Source: Kremer Pigmente GmbH

Specimen Composition: Pb₂SnO₄

Form: Ground pigment powder

History and Significance: A light-yellow pigment that was widely used in oil painting from the Middle Ages to the dawn of the 19th century. Prohibited today because of its lead content.

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 the 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: None

Specimen: Naples Yellow (Kremer Pigmente Ref. 43130)

CAS Registry #: 13510-89-9

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

Chemical Name: Lead antimonate

Source: Kremer Pigmente GmbH

Specimen Composition: Pb₂Sb₂O₇ [and Pb₃(SbO₄)₂]

Form: Ground pigment powder

History and Significance: A yellow pigment used on Renaissance palettes, which is synthesized by heating a lead compound (oxide or nitrate) with an antimony compound (oxide, sulfide, or potassium antimonate). Particle size is in the micrometer range.

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 the 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: None

Specimen: (Accession #01908) Lead white (Kremer Pigmente Ref. 46000), (Accession #01909) Flake white 10 μm (Masters Pigments Ref. 901010), (Accession #01910) Flake white (Sennelier PW No. 108), and (Accession #01911) Cerussite (Masters Pigments Ref. 951010)

CAS Registry #: (Accession #01908, 01911) 598-63-0 and (Accession #01909, 01910) 1319-46-6

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

Chemical Name: (Accession #01908, 01911) Lead (II) carbonate, synth. and (Accession #01909, 01910) Lead (II) carbonate, basic

Source: (Accession #01908) Kremer Pigmente GmbH, (Accession #01909, 01911) Masters Pigments, USA, and (Accession #01910) Sennelier, France

Specimen Composition: (Accession #01908, 01911) PbCO₃ and (Accession #01909, 01910) 2PbCO₃⋅Pb(OH)₂

Form: Ground pigment powder

History and Significance: The most widely used white pigment, progressively replaced during the beginning of the 20th century with newly available less toxic alternatives. A mixture of cerussite (lead carbonate) and hydrocerussite (basic lead carbonate) with proportions varying according to the manufacturing process, which can inform about the origin of the pigment. It is synthesized using lead strips, vinegar, CO₂, and heat. The latter two are usually provided by a fermenting material. In the presence of acetic acid vapors, the lead surface corrodes to lead acetate, which is then converted to basic lead carbonate in the presence of CO₂. This stable process has been used over centuries, its origins dating back to Ancient Greece and China. It was used as a cosmetic since ancient times.

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 the 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: None

Specimen: Minium/Red Lead (Kremer Pigmente Ref. 42500)

CAS Registry #: 1314-41-6

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

Chemical Name: Lead tetraoxide

Source: Kremer Pigmente GmbH

Specimen Composition: Pb₃O₄

Form: Ground pigment powder

History and Significance: This naturally occurring lead oxide has been synthesized since early times by heating lead carbonates. Its color approaches that of cinnabar, providing a cheaper alternative. Used since ancient Greek times as a rust inhibitor for metals, it remained its main use in the 20th century until it was banned in the 1990s.

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 the 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: None

Specimen: Massicot, Litharge (Kremer Pigmente Ref. 43010)

CAS Registry #: 1317-36-8

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

Chemical Name: Lead (II) oxide

Source: Kremer Pigmente GmbH

Specimen Composition: PbO

Form: Ground pigment powder

History and Significance: Litharge and massicot are lead monoxide polymorphs with the formula PbO, respectively, red and yellow-orange in appearance. They are both formed by heating lead or lead white. Litharge plays a key role in the preparation of drying oil for painting: when mixed, it accelerates the polymerization of the film.

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 the 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: None

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°