Decorative layers on tin bronzes

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A R C H I V E S

o f

F O U N D R Y E N G I N E E R I N G

Published quarterly as the organ of the Foundry Commission of the Polish Academy of Sciences

ISSN (1897-3310) Volume 10 Issue 4/2010

153 – 156

29/4

A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 1 0 , I s s u e 4 / 2 0 1 0 , 1 5 3 - 1 5 6 153

Decorative layers on tin bronzes

**M. Nadolski*, Z. Konopka, M. Łągiewka, A. Zyska**

Department of Foundry, Technical University of Częstochowa,

Al. Armii Krajowej 19, 42-200 Częstochowa, Polska *Corresponding author. E-mail address: nadolski@wip.pcz.pl

Received 30.06.2010; accepted in revised form 09.07.2010

Abstract

Decorative layers are decisive for aesthetic value of castings, therefore significant demands are raised towards such layers, e.g. pleasant durable colour, gloss, and smoothness. The work discusses the influence of the type of mechanical working applied to the surfaces of CuSn10 tin bronze castings on the quality and durability of a decorative coating. The scope of the work has included designing and manufacturing of cast samples of tin bronze, mechanical working of the surfaces in order to prepare them for applying coating layers, generating decorative layers as a result of chemical reactions, and the quality assessment and comparison of the obtained coating. The assessment of thickness and continuity of the obtained decorative layers based on metallographic examinations has been presented.

Keywords: Decorative layers, Surface preparation, Artistic casting

1. Introduction

Covering structural materials with various types of protective and/or decorative coatings is one of the most effective methods of corrosion protection and allows for achieving a suitable decorative effect. A wide range of coating types can be used for this purpose, e.g. metal, conversion, inorganic, and organic coatings, being applied to the whole variety of materials such as steels, cast irons, non-ferrous metals and their alloys [1].

The most important demands raised towards the decorative coatings are the pleasant durable colour, gloss and smoothness, along with the tarnish resistance. Surface tarnish occurrence results in gloss detriment and appearing of stains, thus deteriorating the decorative qualities. Achieving of a proper gloss and colour do not exhaust the possibilities of creating decorative effects by means of coatings. Using suitable material sets one can develop some specific types of decorative layers such as hammer finish, mosaic coating, cracked coatings, and dull coating which lustre is lost to a certain degree. As far as bronzes are concerned, the most widely used decorative coatings are the so-called patinas originating as a result of chemical reactions, which allow for providing the surfaces with a wide range of colours [2-5].

First traces of patination can be found already on artworks from the Bronze Age. Bronze mirrors excavated in ancient Chinese tombs reveal the presence of chemical compounds applied distinctly on purpose. We do not know if this was done for stressing the high social status of their owners or for strictly aesthetic reasons to accentuate the beauty of the object itself. However, it confirms that the bronze colourization dates almost as far back as its casting. Traces of artificial patination or colourizing of bronze has been found also in other ancient cultures. Traces of extrinsic pigments which permit to insist that surfaces have been painted with a mix of eggs, water, and pigments have been observed on bronze sculptures in India and Greece [3]. If it has been done for aesthetic or religious purposes, it is still a question.

Not until the High Middle Ages, when alchemists started their researches, the colourization of metals became the object of interest again. This interest, however, was a side effect of searching the way of transforming base metals into gold rather than the result of an aesthetic or historical quest.

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(glaze) colours. A wide spectrum of colours was achieved by using pigments intended for production of lacquers, which in many cases had been designed to imitate translucent patinas obtained by chemical reactions.

At the beginning of the XX century, as the chemistry advanced, a circle of persons specialized in creating chemical decorative layers emerged. This group started a new period to the patinating art, so that today it flourishes thanks to expanding not only the spectrum of colours, but also their richness and texture range. It gives still greater possibilities of surface finishing to the sculptors and other artists, allowing them to express their creativity in the most desirable way [3, 5].

Specified chemical compounds are used for colourization of copper alloys. These are the inorganic compounds containing metals, which can be divided into three categories intended for quite different purposes, and applied either separately or in various combinations. Potassium polysulfide is applied for outer layers, nitrates serve as binders, and both oxides and hydroxides are used to highlight the effect achieved by using nitrates [1-5].

Almost all available recipes recommend using chlorides for obtaining a green coating, nevertheless some sources warn that applying of either ammonia, hydrochloric acid, or chlorides should be forbidden in professional patination since they bring

forth the ‘bronze disease’ – an evil type of corrosion which

spreads quickly and is destructive to bronze. However, experiments with chlorides during the amateur, single trials can be interesting due to the obtained light green colour, brightening the hue of a base.

Both formerly and at present, the quality of patinas is closely related to the way of surface treatment. Different mechanical methods of surface preparation can result in obtaining patinas with various intensity and colour richness.

2. Methods of investigation

The work has been aimed to determine the influence of mechanical preparation of the surface of a CuSn10 tin bronze casting on the quality and durability of its decorative coating. The scope of work has included mechanical preparation of the surfaces of specimens for applying decorative layers, generating of such layers by means of chemical reactions, and the assessment of their quality. Metallographic examinations have been held in order to find the thickness and to check the continuity of the coating.

Examination has been held for specimens of dimensions

50×85×4 mm cast of CuSn10 (B10) tin bronze. The shape and

surface configuration of the specimens after removing the gating system is shown in Figs.1‚5. The surfaces of the achieved specimens have been mechanically prepared before generating the chemical decorative layers. One part of them has been subjected to abrasive cleaning in an air-operated shot-blasting machine using fine silica sand, the remaining part has been manually ground with 150 grit or 240 grit sandpaper. Applying of patinating solutions has been done directly after mechanical preparing of surfaces, without additional degreasing. One set of specimens subjected to the action of a given patinating solution has consisted of one sand-blasted specimen, one specimen ground with 150 grit sandpaper, and one prepared with 240 grit sandpaper.

Patinating solutions have been applied to the specimens heated up to 360 K by either spraying, or painting, or in the end by immersion, according to the recipes presented in Table 1.

The final stage of the experimental work has been the assessment of the obtained decorative coatings. Their colour, structure, and the degree of adherence have been taken into account by rating each category from 1 to 5 with 5 standing for the best. An additional metallographic examinations have been held for the non-etched sections cut out of the sand-blasted specimens.

Table 1.

Components of patinating solutions for 1dm3 H2O

Solution Components Method of applying Patina 1 60g - K2SX spraying

Patina 2 0,4dm3 - (NH)4S spraying

Patina 3 100g - Cu(NO3)2·3H2O immersion

Patina 4 1st layer:

4g - K2SX

2nd layer:

100g - Cu(CH3COO)2

spraying painting Patina 5 1st layer:

4g - K2SX

2nd layer:

5g - K2SX

4g - Fe2O3

spraying painting

3. The report on the obtained results

The results of applying the selected patinating solutions to the prepared specimens made of CuSn10 alloy are presented in Figs. 1-5.

a) b) c)

Fig. 1. Decorative layers on specimens according to Patina 1 recipe. Surface preparation: a) sand blasting, b) 150 grit

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a) b) c)

sandpaper, c) 240 grit sandpaper

a) b) c)

a) b) d)

The thickness and the continuity of the obtained layers has been assessed by observing microstructures presented in Figs. 6, 7 and 8.

a) b)

Fig. 6. Microstructure of decorative layers: a) Patina 1, b) Patina 2

a) b)

Fig. 7. Microstructure of decorative layers: a) Patina 3, b) Patina 4

Fig. 8. Microstructure of decorative layers: Patina 5 The rating evaluation of the intensity and uniformity of the obtained decorative layers and of their degree of adherence to the casting surface depending on the CuSn10 alloy surface preparation is gathered in Table 2.

Table 2.

Results of examination (rating from 1 to 5)

Patin

a Abrasive working

Colour

Adherence to the surface Intensity Uniformit

y

1

Sand blasting 5 5 4

150 grit sandpaper 2 3 1 240 grit sandpaper 2 3 1

2

Sand blasting 4 4 4

3

Sand blasting 4 3 4

4

Sand blasting 3 3 3

5

Sand blasting 4 3 4

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The results of examinations allow for stating that surface preparation by sand-blasting is the most advisable way of surface preparation for all types of patinating solutions used in the experiment. The coatings on the specimens ground with sandpaper are of poor quality as compared with the ones on the sand-blasted specimens, and the patinating process itself is more difficult. Moreover, the patinas exhibit diversified colour and poor adherence to the bronze specimen. The sand-blasted specimen do not reveal such negative properties. High quality of coating layers on the blasted specimens is related to the fact that the sand-blasted surface has the largest roughness, what is additionally confirmed by microphotographs, and therefore the adherence of patina to the metal surface is increased. There is observed a layer at the bronze surface, probably composed of oxides originated between the sand-blasting and the patinating operations. The thickness of this layer can be decisive for occurring difficulties in obtaining the uniform colour of the decorative layer obtained by chemical reaction of reagents with metal.

4. Conclusions

The performed examinations and the obtained results permit to say that the final appearance of decorative layers is influenced to the large degree by the type of mechanical preparation of the patinated surface. Patinas obtained on the sand-blasted specimens exhibit uniform and intensive colour and the best durability for all the examined patinating solutions. As the smoothness of specimen

surface increases, the patinas became both non-uniform with respect to their colour and less durable. Sand-blasting not only provides the desired roughness of surface, but allows also for the treatment of the hard-to-access places, what makes the method even more attractive as compared with other ones. It is also noteworthy that the time of surface preparation by sand blasting is advantageously shorter and improves the efficiency of work. The results indicate clearly that sand blasting is the most advisable method of mechanical preparation of B10 bronze surface for applying chemically originated decorative layers. The patination process should be held as early as possible after the sand blasting, so that the forming oxide layer would be thin enough. The desired final effect depends, however, also on the artistic design.

References

[1] A. Kozłowski, J. Tymowski, T. Żak, Protective Coatings,

PWN, Warsaw, 1978 (in Polish).

[2] W. Skubała, Protective and decorative coatings, Ed. of High

School of Engineering, Koszalin, 1985 (in Polish).

[3] P.V. Kipper, Patinas for Silicon Bronze, Path Publications, Seattle, 1996.

[4] A. Sękowska, S. Sękowski, Metals at our home, WNT,

Warsaw, 1988 (in Polish).