Simple yet Complicated – An Evaluation of Airbrush Technique
air stream atomizes the liquid or paint into tiny droplets as it blows past a very fine paint-meter-ing component. Various triggers are designed to control the amount of liquid, allowing an opera-tor to achieve finely smooth blending effects.
Three factors affect the effects of the airbrush technique – the spray distance, the air pressure and the viscosity of the paint. Generally speak-ing, the airbrush should be placed a distance of 10 to 20 cm from the object to be sprayed. Pres-sure should be adjusted between 0.1Mpa to 0.2 Mpa in general, although it can be reduced to 0.02Mpa to 0.05Mpa in order to achieve fine, deli-cate results. The ideal viscosity of the substance used for spray purposes should be maintained between 45 to 60 cp.
For the purposes of this study, three stages were designed to evaluate the applicability of airbrush techniques using cellulose powders for filling purposes: (1) the type of cellulose powder;
(2) appropriate viscosity of the powder mixture;
and (3) the mixture for spray application.
Selecting cellulose powders
The first stage of this study involved mixing
cellulose powders with cellulose ethers to deter-mine appropriate types of cellulose powders that spread smoothly and will not block the nozzle during spray operation. Cellulose ethers have been studied extensively for their stability (Bac-ker, 1984; Feller & Wilt, 1990). Three different cellulose fibers with 0.2% methyl cellulose added were selected for evaluation. The formula is de-scribed as follows:
1. 0.5 g of Arbocel BC 1000 fiber with 10 g of 0.2%
methyl cellulose.
2. 0.5 g of Arbocel BC 200 fiber with 10 g of 0.2%
methyl cellulose.
3. 0.5g of microcrystalline cellulose with 10 g of 0.2% methyl cellulose.
A spray test showed that the microcrystalline cellulose powder mixture sprayed smoothly with no blocking effect, and it was the only type of cellulose which could pass through the 0.1mm diameter nozzle of the airbrush. The results showed that microcrystalline cellulose was an appropriate type of cellulose for further study as a filler material.
Viscosity test
After the first stage, the microcrystalline cellu-lose powders were then mixed with cellucellu-lose ethers and gelatin. Six groups of mixtures, all containing methyl cellulose (MC), carboxymethyl cellulose (CMC), microcrystalline cellulose (MCC), water and ethanol, were divided into two major subgroups depending whether or not gelatin was added. Among the ingredients, ethanol was used to increase mobility and to speed up the drying rate. Gelatin is used as a sizing agent and as di-spersing agent in the paper industry. The formu-lations and ratio by weight are shown as follows and in Table 1.
A: 2%MC, MCC, water, ethanol B: 2%CMC, MCC, water, ethanol C: 2%MC, 2%CMC, MCC, water, ethanol D: 2%MC, MCC, water, 2%gelatin, ethanol 99%
E: 2%CMC, MCC, water, 2%gelatin, ethanol 99%
F: 2%MC, 2%CMC, MCC, water, 2%gelatin, etha-nol 99%
Measurement of viscosity was then carried out in order to adjust the viscosity of mixtures to between 40 to 60 cp, which will yield better consistency for spraying effects. A viscosity cup was used for measurements. Drain time for this test was suggested as between 17 to 23 seconds
Fig. 2: Before and After Filling
at 25°C ± 2°C, equivalent to 40 to 60 cp at ideal viscosity for airbrush application. The results of the viscosity test are shown in Table 2. The drain time for all tested groups fell in the range of 19 to 23 seconds, which indicates that all of the te-sted mixtures are suitable for spray application.
Mixtures for Spray Test
Different amounts of mixtures were designed for spraying at a distance of 15 to 20 cm. In this test, 1oz and 0.5oz of mixtures were sprayed onto a surface of Xuan paper in size of 5cm by 10cm.
After spraying, roughness of six groups were me-asured by using Mitutoyo Surface Roughness SJ-301. The results of test showed that the roughness values of groups A and C were greater. Group F was shown to be the most consistent of all tested groups. In addition, the evenness of the mixture in Group F also appeared to be more controllable for spray application.
Conservation Implementation
The hand scroll, painted by Lin-can Lee, used for this study was seriously degraded by mold. The treatment process included surface cleaning,
facing, backing removal, first layer of lining, se-cond layer of lining, removing the facing paper, mapping the losses, applying masking to losses, filling losses with airbrush, in-painting and mo-unting. The method used to fill losses using an airbrush is as follows. Strips of Xuan paper were used to cover holes. Water was then applied on the Xuan paper according to the shape of the hole. Water tear was performed to shape areas to be compensated, shown on Figure 1. Based on the result of this experiment, the powder mixture F was chosen to be used as filling material in accor-dance with the airbrush technique. After filling, 0.2% gelatin was also sprayed to provide further reinforcement. Before and after filling are shown in Figure 2.
Conclusion
The results showed that airbrushing can be an ef-ficient way to fill losses of paper. A Chinese hand scroll was successfully restored in a follow-up conservation operation. In conclusion, this study offers an alternative method for filling losses in paper artifacts with special considerations.
Ingredients Groups
Cellulose Binder Solvent
MCC MC
2%
CMC 2%
Gelatin 2%
Ethanol 99%
Water
A 1 1 – – 2 2
B 1 – 1 – 2 3,5
C 2 5 1 – 4 5,5
D 1 1 – 2 2 2
E 1 – 1 2 2 5
F 1 2 1 4 4 3
Table 1. Ingredients of Powder Mixtures (Weight in grams)
Table 2. Results of Viscosity Test
Group Drain time in Average ± Standard Deviation (seconds)
A 20.8 ± 0.8
B 20.2 ± 0.4
C 23 ± 0.9
D 19 ± 0.7
E 19.6 ± 0.5
F 23 ± 0.0
Authors Xing Kung Liao
Conservator in private practice xingkung@gmail.com
Fei Wen Tsai*
Associate Professor, Tainan National Uni-versity of the Arts, 66 Darci Village, Kuentien District, Tainan City, 72045,
*Corresponding author tsaifw@mail.tnnua.edu.tw
References
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