## 2 Materials and Methods
### Fiber Optic Reflectance Spectroscopy (FORS)  
The reflectance measurements were performed using a reflection probe coupled with the QEPro spectrometer (Ocean Optics, Ostfildern, Germany) and a DH-2000 light source (Ocean Optics, Ostfildern, Germany). BaSO\({}_{4}\) was used as standard for calibration. FORS was used to assess the effect of accelerated aging on pigment color, so the spectra were collected before and after aging. Pyrolysis-Gas Chromatography/Mass Spectrometry (Py-GC/MS) and Thermally Assisted Hydrolysis and Methylation (THM) of Py-GC/MS (THM-Py-GC/MS)  
Before and after accelerated aging, small fragments of each sample (ca. 100-110 \(\upmu\)g) were weighed using a microanalytical balance (0.001 mg) and placed in a sample cup (ECO-CUP Frontier Lab, Nagoya, Japan). All samples were investigated with a pyrolizer type PY-2020 iD of Frontier Lab (Japan) and a gas chromatograph/mass spectrometer, GC/MS-QP2010 Plus of Shimadzu (Kyoto, Japan), with a quadrupole mass analyzer. The interface pyrolizer and the injector temperature were set at 280 \({}^{\circ}\)C. The GC/MS unit was equipped with a capillary column SLB-5 ms Supelco (30 m length \(\times\) 0.25 mm internal diameter \(\times\) 0.25 \(\upmu\)m film thickness), using bonded and highly crosslinked 5% diphenyl/95% dimethyl siloxane. Helium was used as carrier gas (constant flow at 0.60 mL/min, split mode). The experimental conditions were slightly different depending on the type of sample. For instance, for the samples containing the acrylic and styrene-acrylic emulsion (Acr_V, Sty_V), no pre-treatment was necessary, and pyrolysis analysis was performed at 600 \({}^{\circ}\)C. The chromatographic conditions for Py-GC/MS for acrylic and styrene-acrylic samples were as follows: the oven initial temperature was set at 40 \({}^{\circ}\)C for 2 min, followed with a gradient of 16 \({}^{\circ}\)C/min up to 220 \({}^{\circ}\)C and 10 \({}^{\circ}\)C/min up to 315 \({}^{\circ}\)C for 2 min. The column was directly coupled to the ion source of the mass spectrometer. The MS interface and ion source temperature were at 280 \({}^{\circ}\)C and 200 \({}^{\circ}\)C, respectively. Ions were generated by electron impact (EI) ionization in positive mode at 70 eV. The mass spectrometer was set from \(m/z\) 50 to 650, with a cycle time of 0.2 s.  
For the alkyld and oil samples (Alk_V, Oil_V, and Oil_V_COMM), THM-Py-GC/MS analytical mode was employed. This method was chosen to improve the detection of high-polarity compounds, such as large acids, which are mainly present in drying oils. The use of the tetramethylmonium hydroxide TMAH (25 wt.% aqueous solution, Sigma-Aldrich, St. Louis, MI, USA) reagent reduces the polarity of these compounds. TMAH reacts in contact with the sample and during pyrolysis, converting carboxylic acids and their esters into their respective methyl esters. Therefore, a pre-treatment with TMAH was used for Alk_V, Oil_V, and Oil_V_COMM. In particular, 5 \(\upmu\)L of TMAH was added to the sample for one hour. Moreover, to perform a semi-quantitative analysis, 5 \(\upmu\)L of a 1 mmol/mL FA-C19 solution in methanol was added to each sample cup before placing it in the pyrolizer. The single-shot pyrolysis was performed at 650 \({}^{\circ}\)C.  
The GC oven-temperature program for the THM-Py-GC/MS analysis of alkyld and oil samples was set as follows: 35 \({}^{\circ}\)C (held 1 min)-30 \({}^{\circ}\)C/min up to 110 \({}^{\circ}\)C-15 \({}^{\circ}\)C/min up to 240 \({}^{\circ}\)C and 5 \({}^{\circ}\)C/min up to 315 \({}^{\circ}\)C (held 2 min). For the THM-Py-GC/MS, a solvent cuttime of 5 min by turning off the filament in the ion source was used. This mode prevents the sharp decrease in the vacuum inside the ion source due to the entrance of the TMAH reagent, which has a detrimental effect on the filament and other components. All the other conditions were the same as used for the acrylic and styrene-acrylic samples. For collecting and processing mass spectral data, Chromeleon 7 (Thermo Fischer Scientific, Waltham, MA, USA) software was used.