Lukisan minyak

Lukisan minyak adalah proses mengecat dengan pigmen yang dicampurkan dengan medium yang terdiri dari minyak pengering. Minyak pengering yang biasa digunakan termasuk minyak biji rami, minyak biji madat, minyak jauzah dan minyak kesumba. Minyak ini boleh direbus dengan resin, seperti resin pain atau kemenyan untuk mewujudkan sapang; yang dihargai untuk sifat likat dan licaunya. Minyak yang berbeza memberikan pelbagai sifat kepada cat minyak, seperti kurang kekuningan dan masa pengeringan yang berbeza. Perbezaan tertentu juga dilihat dalam kilauan cat bergantung kepada jenis minyak. Artis mungkin menggunakan beberapa minyak yang berbeza dalam lukisan yang sama bergantung pada pigmen tertentu dan kesan yang diingini. Cat itu sendiri juga membentuk kelikatan tertentu bergantung kepada medium.

Walaupun cat minyak pertama kali digunakan dalam Lukisan Buddha oleh pelukis India dan Cina di barat Afghanistan sekitar antara abad kelima dan kesepuluh,  ia tidak begitu populariti sehingga abad ke-15. Amalan tersebut mungkin telah berhijrah ke arah barat pada Zaman Tengah. Cat minyak akhirnya menjadi medium utama yang digunakan untuk mencipta karya seni apabila kelebihannya menjadi terkenal. Peralihan ini bermula dengan lukisan Awal Netherlandish di utara Eropah, dan dengan kemuncak Renaissance teknik lukisan minyak telah hampir sepenuhnya menggantikan  cat tempera di kebanyakan Eropah.

Pada tahun-tahun kebelakangan ini, cat minyak larut campur air telah menonjol, sedikit sebanyak menggantikan penggunaan minyak tradisional. Cat larut air mengandungi pengemulsi yang membolehkan ia untuk dicairkan dengan air (bukan dengan pelarut cat), dan membenarkan masa pengeringan sangat cepat (1-3 hari) berbanding dengan minyak tradisional (1-3 minggu).

Techniques

Teknik lukisan minyak tradisional sering bermula dengan artis melakarankan subjek ke kanvas dengan menggunakan arang atau cat cair. Cat minyak biasanya dicampur dengan minyak biji rami, alkohol galian gred artis atau pelarut lain untuk mewujudkan cat cair, yang kering lebih cepat atau lebih lambat. Disebabkan pelarut ini melarutkan minyak dalam cat, ia juga boleh digunakan bagi membersihkan berus cat. Satu peraturan asas penggunaan cat minyak adalah 'lemak lebih'. Ini bermakna bahawa setiap lapisan tambahan cat perlu mengandungi lebih minyak berbanding daripada lapisan di bawah untuk membolehkan pengeringan yang betul. Jika setiap lapisan tambahan mengandungi minyak yang kurang, lukisan akhir akan retak dan berkulit. Terdapat banyak media lain yang boleh digunakan dalam lukisan minyak, termasuk lilin sejuk, damar, dan varnis. Media-media tambahan boleh membantu pelukis bagi menyesuaikan lut cahaya cat, kilauan cat, kelikatan atau 'badan' cat, dan keupayaan cat untuk mengekalkan atau menyembunyikan sapuan berus itu. Pembolehubah ini adalah berkait rapat dengan keupayaan ekspresif cat minyak.

Secara tradisinya, cat dipindahkan ke permukaan lukisan dengan menggunakan berus cat, tetapi terdapat kaedah lain, termasuk menggunakan pisau palet dan kain buruk. Cat minyak kekal basah lebih lama berbanding lain-lain jenis bahan artis', yang membolehkan artis untuk menukar warna, tekstur atau bentuk orang. Suatu masa, pelukis mungkin menyingkirkan keseluruhan lapisan cat dan mula semula. Ini boleh dilakukan dengan kain buruk dan sedikit turpentin untuk tempoh masa yang tertentu ketika cat masih basah, tetapi selepas beberapa ketika, lapisan yang keras perlu dikikis. Cat minyak dikeringkan melalui pengoksidaan, dan bukannya melalui penyejatan, dan biasanya ia kering untuk disentuh dalam tempoh dua minggu. Ia biasanya cukup kering untuk divarnis dalam tempoh enam bulan hingga setahun. Konservator seni tidak menganggap lukisan minyak benar-benar kering sehingga ia berusia 60 hingga 80 tahun.

Sejarah

Sungguhpun sejarah tempera dan media berkait di Eropah menunjukkan bagawa lukisan minyak dijumpai di sana secara bebas, terdapat bukti bahawa lukisan minyak telah digunakan lebih dahuli di Afghanistan. Permukaan di luar dan bagi objek seperti perisai — kedua-duanya digunakan dalam pertandingan dan yang digantung sebagai tropi — lebih tahan lama apabila dicat dengan media berasaskan minyak berbanding dengan apabila dicat dengan cat tempera tradisional. 

Kebanyakan sumber Renaissance, khususnya Vasari, mengkreditkan pelukis utara Eropah abad ke-15, dan Jan van Eyck khususnya, dengan "ciptaan" lukisan dengan media minyak pada panel kayu sokongan ("sokongan" adalah istilah teknikal untuk alas belakang lukisan). Walau bagaimanapun, Theophilus ( Roger daripada Helmarshausen ?) jelas memberikan arahan untuk lukisan berasaskan minyak dalam risalahnya, Mengenai Pelbagai Seni (On Various Arts), ditulis pada 1125. Pada masa ini ia mungkin digunakan untuk mengecat ukiran patung, ukiran dan kelengkapan kayu, terutamanya untuk kegunaan luar. Lukisan Awal Netherlandish dalam abad ke-15, bagaimanapun, yang pertama untuk membuat minyak sebagai medium lukisan biasa, dan meneroka penggunaan cat berlapis dan sepuhan, diikuti oleh seluruh Eropah Utara, dan hanya kemudiannya oleh Itali.

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Although surfaces like linoleum, wooden panel, paper, slate, pressed wood, and cardboard have been used, the most popular surface since the 16th century has been canvas, although many artists used panel through the 17th century and beyond.  Panel is more expensive, heavier, harder to transport, and prone to warp or split in poor conditions. For fine detail, however, the absolute solidity of a wooden panel gives an advantage.

Process

Oil paint is made by mixing pigment containing the color with oil.  Different colors are made up, or today bought pre-mixed, before painting begins, but further shades of color are usually obtained by mixing small quantities together as the painting process is underway, traditionally on an artist's palette, a thin wood board held in the hand. Pigments may be any number of natural or synthetic substances with color, such as sulphur for yellow or cobalt for blue. Traditional pigments were based on minerals or plants, and many have proved unstable over a very long period, so that many old paintings look very different to their original appearance. Modern pigments often used synthetic chemicals. The pigment is mixed with oil, usually linseed oil but other oils may be used as well. The various oils dry differently, creating assorted effects.  

Traditionally, artists mixed their own paints from raw pigments that they often ground themselves and medium. This made portability difficult and kept most painting activities confined to the studio. This changed in the late 1800s, when oil paint in tubes became widely available. Artists could mix colors quickly and easily, which enabled, for the first time, relatively convenient plein air (outdoor) painting (a common approach in French Impressionism).  

The artist most often uses a brush to apply the paint, often over a sketched outline of their subject in another medium.  Brushes are made from a variety of fibers to create different effects. For example, brushes made with hog's bristle might be used for bolder strokes and impasto textures. Fitch hair and mongoose hair brushes are fine and smooth, and thus answer well for portraits and detail work. Even more expensive are red sable brushes (weasel hair). The finest quality brushes are called kolinsky sable; these brush fibers are taken from the tail of the Siberian mink. This hair keeps a superfine point, has smooth handling, and good memory (it returns to its original point when lifted off the canvas); this is known to artists as a brush's "snap." 

In the past few decades, many synthetic brushes have come on the market. These are very durable and can be quite good, as well as cost efficient. Floppy fibers with no snap, such as squirrel hair, are generally not used by oil painters. Sizes of brushes also are widely varied and used for different effects. For example, a "round" is a pointed brush used for detail work. "Flat" brushes are used to apply broad swaths of color. "Bright" is a flat with shorter brush hairs. "Filbert" is a flat with rounded corners. "Egbert" is a very long "Filbert" and is rare. The artist might also apply paint with a palette knife, which is a flat, metal blade. A palette knife may also be used to remove paint from the canvas when necessary. A variety of unconventional tools, such as rags, sponges, and cotton swabs, may be used.  Some artists even paint with their fingers.

Most artists paint in layers which are known as "glazes", also simply called "Indirect Painting".  The method was first perfected through an adaptation of the egg tempera painting technique and was applied by the Flemish painters in Northern Europe with pigments ground in linseed oil.  More recently, this approach has been called the "Mixed Technique" or "Mixed Method". The first coat (also called "underpainting") is laid down, often painted with egg tempera or turpentine-thinned paint.  This layer helps to "tone" the canvas and to cover the white of the gesso. Many artists use this layer to sketch out the composition.  This first layer can be adjusted before moving forward, an advantage over the 'cartooning' method used in Fresco technique. After this layer dries, the artist might then proceed by painting a "mosaic" of color swatches, working from darkest to lightest. The borders of the colors are blended together when the "mosaic" is completed. This mosaic layer is then left to dry before applying details. 

Artists in later periods, such as the impressionist era, often used this Wet-on-wet method more widely, blending the wet paint on the canvas without following the Renaissance-era approach of layering and glazing. This method is also called "alla prima".  This method was created due to the advent of painting outdoors, instead of inside a studio.  While outside, an artist did not have the time to let each layer of paint dry before adding a new layer. Several contemporary artists use a blend of both techniques, which can add bold color (wet-on-wet) as well as the depth of layers through glazing. 

When the image is finished and has dried for up to a year, an artist often seals the work with a layer of varnish that is typically made from damar gum crystals dissolved in turpentine.  Such varnishes can be removed without disturbing the oil painting itself, to enable cleaning and conservation. Some contemporary artists decide not to varnish their work, preferring that the surfaces remain varnish-free.
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