Identification of metal threads from Croatian fabrics

The purpose of the metal threads in the fabric from the beginning of their application has been to show power, wealth, and honour. In ceremonial folk costumes, they signified a social role or had a ceremonial, ritual, or some other function. Liturgical clothes were decorated with shiny metal threads, which were worn by the priests during the Mass. By analysing historical fabrics, metal threads had a decorative character in order to obtain luxury items such as liturgical vestments, clothing for the upper class, upholstered furniture, and curtains. Recently, metal threads of lower metal values have been used in technical fabrics as additional composite reinforcements, protection against solid electricity, or electromagnetic radiation [1].

Metal threads appear in two forms, as alone or wrapped around a non-metal textile yarn. Textile-metal yarn (“srma”) is obtained by spiralling a metal thread around a non-metal textile thread that forms the core. Non-metal textile yarns can be made of cellulose fibres (linen, cotton, hemp) or protein (silk, wool). In the second half of the twentieth century, synthetic fibres also appeared, most often in mixtures with natural ones. The first finds of metal threads were gold narrow stripes. However, they were fragile, difficult to work with, and very expensive, so they were soon replaced by silver- and gold-plated ribbons, which are more complicated to produce than those made of pure metal. Aluminium began to be used in the twentieth century; its shine can completely imitate silver, and a special process can give it a golden colour [2,3,4,5,6].

It is assumed that the technology of making “srma” developed in China and spread to the Middle East. From the seventh to the tenth century, the Arabs spread it across the Mediterranean. The oldest known specimen dates from the late ancient times of Egypt and is kept in the Victoria and Albert Museum in London. The first “srma” workshops existed on the island of Cyprus, from where it was used and spread throughout Europe around 1000 [7]. Cypriot gold is a membrane thread, gilded skin or gut of an animal, which is wrapped around a fibrous core. Membrane threads of Chinese or Japanese origin usually had a paper backing. Membrane threads are more flexible and lighter than solid metal threads, but are of poorer quality due to the fragility of the gold layer towards abrasion and lower gloss compared to solid metal threads. By the thirteenth or fourteenth century, the production of membrane threads also developed in Europe, but their importance declined until the sixteenth century [3]. In the area of Croatia, garments have been decorated with metal threads since the Middle Ages, the period of the Old Croatian state, as evidenced by the grave finds of reticulated creations. These remains speak of early use as well as trade of the Croatian Kingdom in the wider Mediterranean area [8]. Fabrics decorated with various types of metal threads are continued until the modern period on clothing and other textiles, primarily for liturgical use, then on the clothes of the bourgeoisie and noble class, ceremonial folk costumes and flags. Shiny metal threads in weaving or embroidery are found on women’s and men’s costumes of the Adriatic, Dinaric, and mostly Pannonian ethnographic zones. Eastern Croatia is best known for its gold embroidery technique, and the gold embroidery of the county and Vinkovci region, Brodsko Posavlje and the particularly prominent gold embroidery of the Đakovo region are different. Craft, tailoring embroidery, originally from Turkey, is found on women’s and men’s folk costumes of the Adriatic and Dinaric zones. It is a fine embroidery made of gold, silver, and silk ribbons on silk, velvet, and wool fabrics [1]. Metal threads are also found on the fabric in the form of ribbons, so-called braid (a decorative woven ribbon that is sewn on clothes as an ornament and fixing the edges). Ribbons with gold and silver threads are still used in making replicas of folk costumes, decorating and making new fashionable clothing, liturgical vestments, on military uniforms for marking ranks, as well as on decorative fabrics [9]. Zdenka Lechner processed various embroidery and weaving techniques with metal threads, noticed differences in the structure and quality of threads, and determined the time limit for the appearance of embroideries with gold threads in Slavonia in half nineteenth century [10]. Janja Juzbašić has published several works on the topic of gold embroidery, and in the book “Slavonian gold embroidery” she presents the entire traditional technique of gold embroidery in Slavonia [11]. The oldest metal threads used in fabrics were metal stripes (Figure 1a), followed by metal wire (Figure 1b). Because the independent metal threads are rigid, difficult to bend, and inelastic, they created a certain discomfort in clothing, when in contact with textile threads, they damaged them and thus reduced the durability of the fabric. Also, due to its higher specific weight than textile non-metal threads, the garment became heavier with higher proportion of metal threads [12]. Metal threads (wires) were used in weaving in order to achieve the lowest possible surface mass of the fabric, bending, and toughness, and it was necessary to thin them as a preparation for weaving. The technique of extraction is carried out by pulling through a series of holes which reduce the diameter, and a gradual reduction in thickness was achieved. After each pass, the length of the wire increases and the thickness decreases [13].

Figure 1

In order to reduce these negatives of fabrics with metal threads (Figure 1a and b) and at the same time to keep their presence in the fabric and to retain the beauty and value they provide, special kind of threads appeared. Those are threads in a combination of textile non-metal thread (core) and metal thread on the surface called “srma” (Figure 1c) [4].

The technology of making threads has changed with technological developments, as well as the use of lighter and cheaper materials, which in their appearance imitate gold or silver [14]. The use of gold-plated and silver-plated threads greatly contributed to the economy of production and to the reduction of the price of the final product.

Combined metal–textile thread increased the flexibility of the thread and thus enabled different interweaving in the fabric (Figure 2) [12].

Figure 2

The first “srma” to replace independent metal threads, mostly in weaving, due to increased flexibility, were gilded leather straps. Such threads were of poor quality because the layer of gold was easily removed, and they quickly lost their shine. They were replaced by “srma” made of textile thread wrapped in gold or silver thread, or gilded or silver-plated [12]. The production of “srma” was the first major change in the production of metal threads, although the exact time of its origin is not known. Metal threads became more flexible with the production of “srma” and could be used in textiles more easily and in many ways. The making of “srma” is a family craft, where metal narrow stripes or wires were twisted around the non-metallic core in the Z or S direction by means of a spindle [4].

There are over a dozen types of gold threads on the market today that are described as twisted, ruffled gold, living wire. In addition to the famous gold embroidery, there is also decoration by weaving, i.e. weaving gold and silver threads, called picking gold (Figure 3). This weaving technique introduces a gold thread that shapes geometric or floral motifs and is mostly used to decorate women’s tops and bottoms of shirts, rubies, and braids, and for festive towels used in customs [11].

Figure 3

Investigation of metal threads in historical fabrics was carried out on the territory of the Republic of Croatia. Fabric samples were found in regional museums in the cities that have ability to give samples of metal threads without damaging the material. Samples taken from liturgical vestments are from Varaždin, Zagreb, Novigrad near Zadar and Osijek, while samples of folk costumes are from Zagreb, Sinj, Split, Dubrovnik, and Osijek. A total of 11 samples of metal threads from weaving were found in the Varaždin City Museum. In the Zagreb area, one sample of woven metal thread was taken from the Treasury of the Zagreb Cathedral, while two more samples were taken from the Zagreb Ethnographic Museum. The Novigrad Museum donated two samples of metal threads from weaving, two samples were found in the Sinj Museum, whereas four more were found in the Ethnographic Museum in Split. In the Ethnographic Museum Dubrovnik four samples were found, whereas in the Slavonian area, the Museum of Slavonia Osijek (MSO), three samples from liturgical vestments and four samples from folk costumes.

2.1

Materials

The materials used in this article are historical textiles, various items of liturgical vestments (Table 1), and folk costumes (Table 2), from which samples of metal threads and “srma” were selected for analysis. The objects of liturgical vestments from which samples of metal threads were extracted are antependia, chasubles, mitres, shoulder straps, covering for the chalice, etc. The objects of folk costumes from which samples of metal threads were extracted are aprons (Figure 4), dresses, vests, shirts, scarves, etc.

Table 1

Liturgical vestments sampling site

Museum	Sample	Sampling site
Treasury of the Zagreb Cathedral, seventeenth century	Narrow stripe	Miter
Varaždin City Museum, seventeenth–nineteenth century	7 srma, 2 narrow stripes, 1 wire	10 samples from decorative ribbon
	1 srma	Sample srma from the cover for chalice
Novigrad Museum, nineteenth century	srma	Dalmatic
	srma	Bag
MSO, eighteenth–twentieth century	Wire	Chasuble
	srma	Antependium
	srma	Antependium

Table 2

Folk costumes sampling site

Museum	Sample	Sampling site
Zagreb Ethnographic Museum, nineteenth–twentieth century	Wire	Belt
	srma	Shirt
Museum Sinj (time period unknown)	Narrow stripe	Vest
	srma	Coat
Ethnographic Museum Split, nineteenth–twentieth century	srma	Coat
	Narrow stripe	Dress
	srma	Dress
	srma	Apron
Ethnographic Museum Dubrovnik, nineteenth–twentieth century	srma	Men vest
	srma	Men vest
	srma	Men vest
	srma	Men vest
MSO, nineteenth–twentieth century	srma	Shirt
	srma	Women belt
	srma	Apron
	srma	Apron

Figure 4

By carefully separating only the hanging threads from the fabric, relatively small samples of independent metal threads (lamellae, wires) and “srma” were obtained. Given the diversity of historical fabrics, as well as their current state, the samples of metal threads separated for testing are of different sizes (3–6 cm), and thus limited in the investigation. The composition of metal threads changed over time, so two groups of patterns were observed over time on metal thread samples in historical Croatian textiles: one is from the seventeenth and eighteenth centuries and the other from the nineteenth and twentieth centuries. Seventeenth- and eighteenth-century specimens are richer in gold and silver and have some copper. Newer samples of metal threads from the nineteenth to the twentieth century are mostly copper or from its alloy with zinc and less often nickel. The metal thread samples included in these analyses were not divided by time stages.

2.2

Methods

The isolated samples were analysed with scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX) device, and the metal content on the surface and in cross section was determined. It was found that the cross section of metal threads can be obtained easily by bending the thread at an angle of 90°. First, the metal thread was cut to a suitable length for measurement than bend at a right angle so that the cut part remains open towards the direction from which the electron beam from SEM-EDX is coming. In this way, the cross section is visible as the surface that the SEM device scans. Due to the stiffness of the metal thread, it remains in a bent position, so there is no need to fix the thread with resin. In this way, composition and the structure of metal threads were determined. Some threads are alloys, and some are in two layers, for example silver-plated or gilded threads. There are also threads from pure metal such as copper or silver metal threads. The operation of the SEM-EDX device is based on scanning the surface of the tested sample with a focused beam of electrons, which is used to examine the appearance of the sample. It is used to study surface structures, and an additional device is used for determining the chemical composition of materials [15]. When a sample is bombarded by electron beams, electrons are ejected from the atoms contained in the sample surface. The resulting voids are filled by electrons from the second higher energy electron shell, and one quantum of energy or X-ray is emitted during this jump, in order to balance the energy difference between the two electron levels. X-ray energy is inherent in the elements from which it is emitted. The energy dispersive spectrometer (EDX) detector detects this type of radiation. Characteristic X-rays are collected by an EDX detector, counted and sorted based on energy. The resulting spectrum has the number of X-rays on the vertical axis, whereas it has energy on the horizontal axis, and the peaks on the spectrum correspond to the elements present in the samples. The number of signals at a certain peak of the spectrum indicates the proportion of the element located in the analysed part of the sample [16]. Secondary electrons are electrons of atoms ejected when interacting with primary electrons from the beam and have very low energy (less than 50 eV). Due to their low energy, they can only pop out of the shallow part of the sample surface, resulting in the best image resolution. The range of interaction is closer to the sample surface, and therefore more electrons can be detected, those from points at the tops of the relief, but also those at the bottom of the relief valley. The peaks of the relief are shiny and the valleys dark, which is why the resulting image looks like it was obtained visually. Backscattered electrons (BSEs) are the primary electrons from the beam that bounced off the sample surface due to elastic interactions with the nucleus of the sample atom. They have high energy, ranging from 50 eV to the voltage of accelerated electrons from the beam. Their higher energy also results in a greater range of interaction and degradation of the resolution of the obtained image. Images obtained from BSEs provide important information about the composition of the sample (Figure 5) [17].

Figure 5

It can be seen that the samples of metal threads meet these conditions for simple and fast analysis, and no special sample preparation is required before analysis. Organic residues on the sample can increase the carbon signal and produce inaccurate quantitative results. The detector on the SEM-EDX is silicon based so if the percentage of silicon is less than 1% it is not part of the sample [18]. In addition to the surface analysis, a cross-sectional analysis (of the interior of the metal thread) was performed in order to determine whether they were alloys or gilded samples, using the method shown earlier [17]. For each sample, the surface and transverse analysis was performed at several points and their mean value was calculated. Light microscope was used for non-metal textile part of srma and it was the Olympus cx22 light, upright and transmission microscope, intended for educational as well as laboratory purposes. It has a built-in revolver with four places for the lens and a halogen lighting system. The microscope is suitable for the analysis of the raw material composition of non-metallic textile yarns in “srma.” The analysis was performed under a magnification of 10× eyepieces, 10× lenses, and the total magnification was 100× [19].

Analysis results of metal threads from liturgical vestments samples are shown in Table 3 and those from folk costumes are shown in Table 4. From the 33 total samples, 17 samples are from liturgical vestment and 16 samples are from folk costumes. Most of the samples are “srma” (25), lamella (5) samples and only 3 are wires. Twelve “srma” samples have a non-metal cotton part, silk appears in eight samples, and five samples have flax for their core. Flax appears only in samples from Varaždin. Silk is mostly found on srma from liturgical vestments and these metal threads have more gold and silver.

SEM-EDX spectra of one pure copper and one pure silver sample are shown in Figures 6 and 7.

Figure 6

Figure 7

There is a compatibility of the metal and non-metal components of “srma” in such a way that most shiny “srma” have more gold and silver in their composition and have silk for the non-metal component, while most copper have “srma” that have cotton or linen in the core. The analysis confirmed the assumption that the older threads from the seventeenth and eighteenth centuries are more valuable, richer in gold and silver, whereas the newer metal threads from the nineteenth and twentieth centuries have more copper in their composition.

Studying the metal threads collected for this article, it is observed that in addition to the morphological differences in the threads, there are also large differences in composition so that they can be classified into three different groups of threads: 1. pure metal threads, silver or copper; 2. different alloys; and 3. two-layer having a surface layer of gold, silver, or their alloys. Among the samples, threads made of pure silver or copper are the least, as they darken over time and lose their shine. There are a few more alloys, and these are mostly alloys of copper and zinc, the so-called brass, because they are more resistant to corrosion. Most of the threads are double-layered, they have some kind of gilding. Most often, the base metal is silver or copper coated with a nobler metal, gold or silver, which gives gilded silver or silver-plated copper. However, copper coated with an alloy of gold and silver was also found, as well as a gilded alloy of silver and copper, but much less frequently. Non-metal components of “srma” that make up the yarn core are cotton and silk at the most, but also flax less often. Silk is always found in samples of “srma” that have a metal thread of gilded silver or pure silver. Cotton and linen are non-metal threads that have copper or copper alloys for the metal component. Silver-plated copper in “srma” is found in combination with cotton, linen, and silk, depending on the location of the sample on the item. The non-metal part of the “srma” is followed by metal in quality, but also in colour. Thus, gold metal threads have yellow non-metal textile threads for the core, whereas silver ones have white. Samples of metal threads are mostly “srma,” less often lamellae and wires.

Analyses of the metal thread composition contribute to the implementation of a scientific approach in the restoration and conservation of historical textile objects.

A new method of cross-sectional analysis for metal threads has been improved and confirmed, given their stiffness and the possibility of bending at right angles, which is simple and adapted for the SEM-EDX device. Based on the analysis of the surface and cross-section, it is possible to determine which samples are homogeneous (alloys) or layered (gilded, silver-plated).

Older metal threads are mostly from liturgical vestment and have more gold and silver in their composition. Newer metal threads from nineteenth and twentieth century are mostly from copper and their alloys with zinc and nickel.