3.3.1. NMR

29Si, 13C, and 1H-NMR spectra in solution were collected using a Bruker AVANCE III 500 MHz spectrometer (Probe BBO-S2 5 mm). The internal references used were TMS (0 ppm) and hexamethyldisiloxane (chemical shifts at 6.54, 2.90, and 0.04 ppm in D2O, and 6.54, 1.96, and 0.06 ppm in DMSO-d6). In the case of 29Si, the one-dimensional sequence and inverse decoupling with a 90◦ pulse was used (d1 from 2 to 5 s, dt = 30 ms, ds = 4, and ns = 512). The 13C-NMR spectra were recorded using the one-dimensional sequence and

proton decoupling with a 30◦ pulse (d1 = 1 to 2 s, dt = 30 ms, ds = 4, and ns = 128), and 1H spectra were obtained with the one-dimensional sequence with d1 = 1 s, ds = 2, and ns = 16.

The 29Si MAS and CPMAS and 13C CPMAS NMR spectra were collected using a Bruker AVANCE III 400 MHz spectrometer (probe: HRMAS 4 mm) using talc (−90 ppm as the reference for 29Si) and adamantane (28.46 and 37.52 ppm for CH and CH2, respectively). The parameters for the chitosan 13C CPMAS experiment were ns = 4096 and d1 = 4 s; for the THEOS/chitosan hybrid 13C CPMAS experiment, ns = 8192 and d1 = 4 s; for the THEOS/chitosan hybrid 29Si CPMAS experiment, ns = 8192 and d1 = 4 s; and for the THEOS/chitosan hybrid 29Si MAS experiment, ns = 14,336 and d1 = 6 s. The samples were placed in 4.0 mm zirconia rotors with a spinning rate of 8 kHz.

#### 3.3.2. FTIR Analysis

The spectra of hybrid films from 4000 cm<sup>−</sup><sup>1</sup> to 650 cm<sup>−</sup><sup>1</sup> were collected using a Perkin Elmer Spectrum FTIR 1600 coupled with an ATR accessory (germanium point, 100 μm in diameter). An average of 16 scans was obtained, with a resolution of 4 cm<sup>−</sup>1. Similar experimental conditions were used in the case of treated and untreated stone samples. The FTIR spectra were obtained from powders (−100 mesh) of each stone.

#### 3.3.3. Scanning Electron Microscopy (SEM)

A palladium–gold alloy was vacuum evaporated on the dried samples. The outer surfaces of the treated stones were then studied using a EVO15-HD ZEISS scanning electron microscope at a 15 kV accelerating voltage under various magnifications (1000, 5000, and 10,000).

#### 3.3.4. Stone Materials and Treatment

Samples of partially decayed siliceous stone (pink tuff) with the measurements 5 cm × 3 cm × 1 cm were collected from three different monuments or locations; two of them correspond to a tuff with a siliceous composition and are from two different historical monuments located in the city of Guanajuato, México (UNESCO World heritage City since 1988). The first is from the basement of a middle 20th century monument, called the statue of General Sostenes Rocha (who fought against the French army in México in the second half of the XIX century). Mineralogical analysis and XRD showed that the composition of the stone is mainly alkaline feldspars (46%), quartz (27%), mica (10%), kaolinite (9%), and smectite (3%), with traces of hematite. The second is from the church known as Oratorio de San Felipe Neri (traditionally called Compañía), which is a religious historical monument from the middle of the XVIII century [33]. The reported mineralogical analysis indicated alkaline feldspar (65%), quartz (29%), calcium silicate (3%), and hematite (4%) contents [44]. Additionally, a third sample from an archeological site called "Cerro de Los Remedios" (located in Comonfort county, Guanajuato state, México) was studied. As part of the basement of a pyramid, this stone possesses a calcareous nature (caliche), with CaCO3 (93%), CaO (1.5%), halloysite (5%), and traces of hematite. The relatively high amount of kaolinite present in the stone might be taken as evidence that some of the original feldspars have been hydrolyzed to clays by the weathering process known as kaolinization [45]. A comparative composition of the samples is presented in Table 7.

To evaluate the consolidation effect of the hybrid formulations, some samples (already cleaned and dried) were treated with the THEOS–chitosan formula, and to test the hydrophobic properties, others were treated with the MeTHEOS–chitosan formulation. The formulations were applied on stone samples by brushing in one phase under laboratory conditions until saturation.

Then, all samples were carefully wrapped in black plastic polypropylene sheets (as used in practical conservation) to permit gelling and aging for 2 weeks.


**Table 7.** Mineralogical composition of the siliceous and calcareous stones.

3.3.5. Characterization of the Samples Treated with the Consolidant and the Hydrophobic Formulations

The hardness changes of the stones untreated and after treatment were measured as the Shore hardness using an REX 2000D indentation durometer. The hardness values obtained were transformed to Vickers, Brinell, and Mohs scales.

Water absorption measurements were carried out using the Karsten (Rilem) pipe [46]. The graduated pipe was fixed onto the sample and filled with water. Water absorption for each sample was measured as the difference between the quantities of water (mL) absorbed after five and thirty minutes.

Static and dynamic contact angles were measured using the OCA 15 Dataphysics system. Contact angle data obtained in the three classical liquids of different polarities (water, formamide, and diiodomethane) were used to calculate the surface free energy or free energy of hydrophobicity. The surface free energy was calculated from the OCA 15 Dataphysics software using the Owens and Van Oss (acid–base) methods [42,43].
