·Table of Contents
·Conservation and Restoration in Art and Architecture
Wax Analysis in conservation objects by solubility studies, FTIR and DSCUlla Knuutinen
EVITech Institute of Art and Design
Waxes are translucent solid substances that melt easily. The source of natural waxes is very diverse: mineral, vegetable and animal. Waxes contain long chain hydrocarbons, acids, alcohols and esters or mixtures of these. Most wax components are fully saturated materials and this results in considerable chemical stability and waterproofing properties. Waxes have been used as adhesives, as painting media, for surface coating purposes, as a component of seals and as a modelling or casting materials. Nowadays waxes have many uses in conservation practice.
The history of detection and identification of waxes starts with simple tests of solubility and melting point. [1,2] These simple tests can be connected to modern methods of analysis to reveal more detailed information about chemical composition and physical properties of waxes. In our research melting point analyses have been made by differential scanning calorimetry (DSC). Solubility tests with changing polarity were used as sample pretreatment before Fourier transform infrared (FTIR) spectroscopy.
Both of these methods (DSC and FTIR) have been used to identify waxes,[3,4,5,6] but in combination DCS and FTIR can provide qualitative characterisation of decomposition processes. Differential scanning calorimetry allows melting point determination and thermal characterisation and FTIR can provide molecular structural information of wax materials.
Differential scanning calorimetry (DSC) can be used to detect the physical or chemical changes in a material that are accompanied by absorption (endothermic) or liberation of heat (exothermic). DSC can quantify the thermal events, if the original mass of sample is known. DSC analyses can be conducted with 0,1 mg of sample, 5-15 mg is a typical size.[7,8]
Fourier - transform infrared (FTIR) spectroscopy is certainly one of the most important analytical techniques available today. One of its great advantages of is that any sample in any state can be studied: gas, liquids, solutions, powders, films and surfaces can be examined by a suitable choice of sampling technique.
The energy at which any peak in an absorbtion or transmission of FTIR spectrum appears corresponds to the frequency of vibration of a part of the sample molecule. This provides information on the functional groups in the molecule. From the frequencies of the absorbtions it is possible to determine whether various functional groups are present or absent. This forms the basis of analyses of the molecular structure of single and multicomponent materials. [9,10]
All DSC samples: wax references (from Kremer Pigments), wax mixtures and unknown samples were weighted out and placed into the thermal analyses chamber (Mettler DSC 20). Via a personal computer and separate thermal analysis processor (Mettler TC 11 TA Processor) the following conditions where set: start 30° C, rate of heating 10° C per minute, and final temperature of 300° C.
These conditions where kept, and the process repeated for all samples.
Before FTIR analysis solubility tests were made as a pre-treatment for reference wax samples. In a test tube few milligrams of wax were placed and solvent was added.
Solvents (pro analyse grade) were chosen to provide a series of a constantly raising polarity: pentane, cyclohexane, toluene, o-xylene, dichloromethane, diethylether, acetone, methanol and ethanol were used as solvents. The samples were left overnight in the fume cupboard.
Two different ways of preparing samples for FTIR were used:
Before measuring samples background was collected with a KBr disc without sample.
Background and FTIR spectra of wax samples with KBr were measured by Nicolet Impact 400 IR.
Results were processed by Omnic version 1.1 by Nicolet Instrument Corporation.
Some samples were examined without extra sample preparation by FTIR microscopy (Mattson) with a diamond anvil cell or were spread over the object glass.
As an example of thermography analyses with DSC some thermography curves (See figs. 1-4 in the text and figs. 1-3 in appendix) will be presented. All the waxes and mixtures show a large through that comes down from the baseline, this indicates an endothermic process. Waxes start softening early in the temperature program. Melting temperature of a highpurity wax is expected to be infinitely sharp (See fig 1,app.) carnauba wax). Also bee wax and unknown wax coating of cupboard, unknown wax wedding decoration (See figs.1-2 and 4, text), and sample candelilla wax (See fig 2, app.) all have one single trough, thus indicating that only one compound is present. They are not mixtures of different waxes.
The two curves representing the bee wax and the unknown sample of the coating of cupboard are identical in the endothermic range.
Fig 1: DSC of bee wax; Fig 1 Appendix: DSC of carnauba wax
Fig 2: DSC of unknown wax coating of cupboard; Fig 2 Appendix: DSC of candelilla wax
Fig 3: DSC of paraffin wax; Fig 3 Appendix: DSC of mixture of carnauba, paraffin, bee wax (20:60:45)
Fig 4: DSC of unknown wedding decoration
Impurities and mixtures broaden the melting point area and give a large exothermic rise towards the end of the program. The paraffin wax (see fig.3) has two peaks, which are clearly distinguishable from each other, thus indicating a mixture of several molecular sizes. The mixture of carnauba, paraffin and bee wax (see fig 3,app.) has a small endothermic trough followed by a large exothermic rise. The melting point area of the mixture is not sharp also indicating thermography curve of a mixture.
In table 1 the measured melting points by DSC are compared to values found in literature. [1, 2, 11,12]
Bee wax melts at about 64°
C and this melting point remains fairly constant with ageing.
Both candelilla, and carnauba waxes are harder than bee wax and their melting points are higher than melting point of bee wax.
As a distillation fraction of petroleum paraffin wax has the lowest melting point and various grades can have different melting point ranges. Paraffin wax has low molecular weight and it usually contains C numbers as low as C 16, which contribute to low melting point. It can release solvents gradually with time causing the wax to become more brittle. This can cause changes in the thermography curve.
|Sample name||DSC / Melting point /°C||Literature/ Melting point /°C|
|Paraffin wax||39.5 and 56.3 (two peaks)||40-65|
|Carnauba, paraffin, bee wax||Between 45 to 80|
|20: 60: 45|
|Unknown wax||48.1||[same as paraffin wax)|
|Unknown wax coating of a cupboard||63.0||(same as bee wax)|
|Table 1 : Melting point results.|
With FTIR spectra it is possible to see a clear difference between mineral waxes and waxes containing lipids, fatty acid esters.  For example mineral wax paraffin contains hydrocarbons only with C-H and C-C bonds. In figure 5, FTIR spectra of paraffin C- H stretching vibrations of saturated hydrocarbons are seen below 3000 cm-1, -CH3 and C-H deformations at about 1460 cm-1 and 1380 cm-1. Rocking and wagging of -CH2- gives a clear peak at 720cm-1.
Figure 5 presents the FTIR spectra of the unknown sample (wedding decoration) using FTIR microscope.
For lipid waxes spectra look more complicated than spectra of the minerals. They include the same bands of long hydrocarbon chains as mineral waxes: C-H stretching vibrations at about 3000 cm-1, C-H bending at about 1470 cm-1 and twin bands at 720 cm-1and at 730 cm-1. The main difference is the presence of a peak in region of 1700 cm-1 which corresponds to a carbonyl (C=O) stretching vibration from free carboxylic acid and from esters.
Fig 5: FTIR absorbtion spectra of unknown sample (wedding decoration)
The infrared spectrum of bee wax (see fig. 6) is rather reliable and constant characteristics useful for the analysis because the spectrum of bee wax changes only little through oxidation with ageing.
In figure 6 FTIR spectra of pure bee wax and in figure 7 spectra of unknown sample (wax coating of a cupboard) are shown. Same bands as in pure bee wax can bee seen in unknown sample. But there are some extra peaks that don't belong to bee wax. This extra material will need further investigation with gas chromatography (GC).
Fig 6: FTIR transmission spectra of bee wax.
Fig 7: FTIR transmission spectra of unknown sample (Wax coating of a cupboard).
Differential scanning calorimetry can be used for wax characterisation at sample level starting from about 0,2 mg. The experiment is very simple without any sample preparation.
The shape of the thermal curve and the melting temperature permit the identification of pure wax samples. The curve also can give information of impurities and reveal wax mixtures, but it doesn't provide any detailed information referring to chemical composition.
Fourier transform infrared spectroscopy in combination with DSC is a very useful technique for chemical identification of waxes. FTIR provides general characterisation of mineral waxes and waxes containing lipids. Infrared serves to identify both paraffin and bee wax.
The most specific chemical information on waxes and wax mixtures can be attained by gas chromatograpy. Further research must include the use of GC, which allows the examination and identification of wax mixtures.
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