Detail from the Westminster Retable

Research into England's oldest medieval altarpiece – which for centuries provided the backdrop to Westminster Abbey coronations – has revealed that it cost no more than the rather unprincely equivalent of eight cows.

Cambridge conservation scientist Spike Bucklow uncovered the knock-down cost of the 1260 AD 'Westminter Retable' while researching his latest book 'Riddle of the Image', which delves into the materials used in medieval works of art.

Commissioned by Henry III during the construction of Westminster Abbey, the altarpiece's use of fake gemstones is already well documented. However, what has not been known until now is just how little the king would have paid for the Retable, the oldest known panel painting in England.

Using centuries-old records of accounts from Westminster Abbey, Bucklow was able to determine prices for the amount of wood used, the area of glass needed, each pigment of paint, and the wages the carpenters and painters were paid. This information was combined with practice-based research into the Retable whilst it was being restored at the Hamilton Kerr Institute.

"This is bargain basement stuff, it was all dirt cheap," he said. "While some of the other objects in Riddle of the Image would have been cost the same as a farm or country home, the Westminster Abbey altarpiece would have cost no more than eight cows or about £5 in 13th century money.

"Historians have often thought that a financially constrained Henry was cutting corners, but you don't spend as much as he did on the rest of the Abbey and then cut corners on the most visual and most important area for the crowning of monarchs."

Rather than penny-pinching to preserve pounds, crowns and shillings, Bucklow believes that Henry III deliberately chose cheap materials and fake gemstones to accentuate one of the key themes of the altarpiece – miraculous transformations.

"It is no coincidence that all three surviving painted scenes show Christ involved in a transformation. Transformation is key to the whole Retable. It was the backdrop for transformations in a very real sense. In front of it, once in a generation, someone was turned into a monarch, while much more often, bread and wine were transformed into the body and blood of Christ.

"To make a fake gem you take sand and ash and transform something ordinary into something beautiful. Henry is telling us that art is above gold. We know how engaged he was with artists of the day. I really believe that he was dedicating human ingenuity and skill to God. He's making a statement."

As well as determining the cost of the Westminster Retable, The Riddle of the Image is an attempt to look at medieval works of art through the eyes of those who commissioned and made them. Bucklow believes that our modern-day appreciation of cultural artefacts – such as mobile phones – is completely divorced from our understanding of the materials that go into their making.

In medieval times, however, there was a widespread knowledge of artists' materials that contributed deeper meaning to objects such as the Metz Pontifical (c.1316) and the Macclesfield Psalter (c.1330), both beautiful illuminated manuscripts now in the Fitzwilliam Museum, as well as the Thornham Parva Retable, which was also restored at the Hamilton Kerr Institute, and the Wilton Diptych, Richard II's iconic portable altarpiece.

Bucklow believes this is because many of the pigments and materials used in the pre-modern world for artistic purposes also had common, everyday uses such as cochineal and lapis lazuli being used in make-up and medicine. (Red dyes were used in heart tonics and the blue stone was used to 'dispel melancholy' and lower fevers.) As such, artists' materials were readily available from apothecaries of the day.

By examining the science of the materials, as well as the techniques of medieval artists, Bucklow hopes to further the reader and art-world's understanding and appreciation of the paintings, and medieval art in general.

Each chapter in the book is devoted to one of five objects and each builds on the cultural relevance of materials, exploring the connections between artists' materials and their everyday life; showing how materials could be used philosophically and playfully.

For example, in one of the book's featured artworks, two blues, one of which cost ten times as much as the other, were used side by side, even though they could not be told apart with the naked eye. In another manuscript, the strange choice of materials matched the bizarre contorted hybrid figures seen swarming across the page margins.

The Riddle of the Image, published by Reaktion Books, is available now.