Andy Warhol and Keith Haring repeatedly opted to engage special materials: tiny glitter particles, powdery chalk, iridescent copper powder or bright neon colours give the works fascinating visual effects. However, due to their fragility and impermanence, these materials pose major challenges for long-term preservation. In the blog post on the occasion of the 7th European Day of Conservation-Restoration, Isabel Gebhardt and Michaela Tischer, conservators at the Doerner Institut, shed light on the specific approaches that are necessary in museum practice to preserve these extraordinary works of art. 

Andy Warhol is known for his iconic motifs, which he perpetuated on painted canvases using silk screen printing.  Unusual materials were sometimes used for this purpose. The "Diamond Dust", with which Warhol worked from 1979 onwards, is a sparkling example. The material designation suggests that these are real diamond particles, which seems quite plausible in view of Warhol's penchant for glamour and expensive jewelry. However, technical investigations revealed the "Diamond Dust" as ground glass. 

Warhol's first attempts to refine motifs on canvas with real diamond dust failed – the tiny particles did not reflect and the desired glitter effect failed to materialize. Instead, Warhol resorted to cheap glitter made of ground glass, commonly used in shop window decorations. As ordinary as the source material is, it makes his works sparkle – including the prominent "Diamond Dust Shoes". 

Through the process of screen printing, Warhol transferred motifs from templates – mostly photographs – to canvases painted with acrylic paint. Immediately afterwards, glass particles were sprinkled into the still wet silk screen ink. However, it is precisely this special choice of material and texture that pushes conservators of modern and contemporary art to their limits. The extremely sensitive surfaces are highly susceptible to dust deposits, which adhere particularly well to the rough structure. Surface cleaning is therefore practically impossible. This makes preventive measures, such as dust-proof storage packaging, an all the more important focus – so that the works can continue to sparkle. 

In New York in the early 1980s, Keith Haring discovered unused billboards in subway corridors. On the black placeholder papers glued into frames, he drew his characteristic "Subway Drawings" with simple blackboard chalk. The drawings delighted and irritated passers-by with their unmistakable visual language and achieved cult status in a very short time. Since the ephemeral chalk drawings hung unprotected in public spaces, only a few of the original estimated 5000 works survived. Today, six of them have found their way into the Brandhorst Collection, where conservators from the Doerner Institute are ensuring their preservation.

The “Subway Drawings” that are still preserved today were often cut out of the frames by Haring´s fans. In some cases, even the entire frame construction made of glass-fibre reinforced plastic was levered off the wall, which left unmistakable traces such as holes in the paper and frame. The genesis and work history of the "Subway Drawings" require a special conservation approach: Contrary to classic conservation practice, dirt from the subway corridors and signs of dismantling are deliberately left, because they tell of the eventful history of the works. 

Warhol´s legendary "Oxidation Paintings" are probably his most experimental series of works. At the end of the 1970s, the artist began to explore the chemical reaction between metal and human urine with the utmost meticulousness and alchemical joy.

The self-mixed paint made of binder and copper powder was spread on the canvases and immediately afterwards brought into contact with urine. The chemical reaction between copper and urine led to oxidation – the namesake for these works, which were often referred to as "Piss Paintings". Intriguing changes to the surface, producing powdery textures and colours ranging from bright green to blues and browns, emerged. Not only was the artist's urine used, but assistants and visitors to the Factory were also invited to immortalize themselves on the shiny metallic surfaces.

The complex processes of oxidation are by no means finished today: they remain active and slowly change the appearance and condition of the paintings. Conservators of modern and contemporary art are concerned with researching and forecasting these changes – alterations that are sometimes intended by artists, but often due to chance or unstoppable aging. In the case of this series of works, it was shown that climatic fluctuations rapidly accelerate the oxidation and lead to striking colour changes. This makes it all the more important to plan the conditions for presentation and storage with particular care and to keep a watchful eye on the "Oxidation Paintings". 

Daylight fluorescent paints, better known as neon paints, have had a lasting impact on the art world since their inception. These bright paints were already very popular with Andy Warhol, as they fit perfectly with the colourful, loud aesthetics of Pop Art. In the 1980s, Keith Haring also used these intense pigments to bring his characteristic figures and symbols to life.  

These special paints consist of fluorescent dyes dissolved in a binder. Their trademark is luminosity, which is most evident in daylight. The term "daylight fluorescence" already indicates this: Even visible light can stimulate the pigments to fluorescence, which gives them a remarkable luminosity.

Despite, or perhaps because of, their visual fascination, these pigments pose challenges: their low lightfastness makes them sensitive to visible light and UV radiation, which can lead to rapid fading of the paint. This sensitivity confronts conservators and museums with the task of finding a good balance between presentation and immediate experience of the art, and its long-term preservation. Special conservation requirements with regard to illuminance and exhibition duration are therefore necessary in order to preserve the luminous painting and to ensure its intense effect for future viewers.