Minerals are solid substances with characteristic chemical composition and internal structure. In nature, they are formed deep in Earth’s interior by precipitation from melt (e.g. diamonds), by crystallisation from hydrothermal solutions (e.g. quartz, claystones) or directly from gases near volcanoes (native sulphur). Mineral crystallisation can be easily observed in the warm summer months, when salt or halite are precipitated from seawater. All of the examples listed are results of crystallisation of inorganic matter. Up to the present day, more than 3,500 different representatives of this class have been identified. They form the solid Earth’s crust and represent an abundant source of metals and other substances that humanity makes good use of. However, crystals formed from organic matter are just as, if not more important for our lives. They are not as easily recognisable though. A very unique environment for the formation of crystals from organic substances are wine barrels. Various minerals that form during wine maturation are collectively known as tartar.

Tartar is the name for potassium and calcium salts which are present in every naturally produced wine. They are released in colloidal form early in the wine-making process, during alcoholic fermentation of the must. This happens to a particularly large extent in cases of rapid fermentation at high temperatures. As the concentration of alcohol rises during fermentation, the solubility of tartar decreases, which leads to the formation of crystals.

Tartar most often refers to potassium bitartrate, which forms when carboxyl groups in tartaric acid combine with potassium molecules. This crystal growth occurs on the barrel walls until large aggregates are formed. Under a microscope, they look like non-uniform elements with rough surfaces and uneven edges. They are partially transparent, with smaller ones being white in colour, while the larger ones take on a brownish hue.

Wine stabilisation

Before bottling, wine stabilisation needs to be performed in order to avoid precipitation of tartar crystals, which would otherwise inevitably occur either due to drops in temperature, exposure to sunlight (UV rays) or mechanical vibrations during transport or bottling itself. In chemical terms, the aforementioned factors disrupt the equilibrium of the solution, resulting in a decrease of solubility. To prevent this, several methods of wine stabilisation exist, among which “physical” methods are mostly preferred, such as cooling, the use of ion exchangers, reverse osmosis and/or membrane electrodialysis.

Another major problem in winemaking is the hard-to-dissolve calcium tartrate, which is precipitated only after bottling. Under a microscope, crystals of calcium tartrate can be distinguished from potassium bitartrate crystals by their large transparent rhomboidal bodies with smooth surfaces and straight, sharply defined edges. They do not coalesce, but instead grow into separate large and transparent crystals.

Crystals in bottled wine often result in consumers thinking that the wine is low-quality. Even a small amount of precipitated crystals visible to the naked eye can cause excessive health concerns or at least misinterpretations. Because of this, winemakers go to great lengths in order to prevent the formation and precipitation of tartar crystals (potassium bitartrate) in wine bottles. But no matter how much tartar we manage to remove from a young wine, it is quite natural for unstable substances to be precipitated after a while. In essence, these are just harmless sediments which consist of tartar, colour additives and some proteins. It would be quite strange and unnatural for sediments not to be present in archival wines.

Tartar in alchemy

The beginnings of alchemy date back to the first centuries AD in ancient Egypt. It was not until the Middle Ages however that alchemy really came to prominence, with numerous alchemical laboratories springing up across medieval Europe. These places played an important role in laying foundations for later scientific developments. Alchemists spread knowledge in various fields, from metallurgy to the production of alcohol. They promoted a holistic approach to science, incorporating both the material and the spiritual sphere. In many aspects, alchemy can be viewed as the progenitor of chemistry, medicine and other modern sciences. Today we know of alchemical experiments from descriptions that alchemists left in recipes, which were sometimes very unusual. Often, tartar (tartrate) was listed as one of the raw materials. It was used as an additive in various metal processing techniques (e.g. silvering of metal surfaces). Tartar also figured in recipes for various healing concoctions. Similarly, even today tartrates are an integral part of some modern pharmaceutical preparations.

Notes on selected exhibits:

White crystals: Malvasia wine matured in 10,000 litre round barrels for approximately ten years. This white wine variety is predominant in the wine-producing region of Koper. Crystallisation of white crystals took at least six months.

White base and red crystals: Initially, the barrel contained Malvasia, which was matured for at least 5 years. Afterwards it was emptied and a red variety from the same region, Refošk, was stored in it. This caused the top layer of crystals to gain a red colour due to natural wine colourants called anthocyanins. The crystals are not as clear and lost some of its lustre due to the presence of sediment, which is the result of the self-clarification process of the wine. Since the stave was located close to the bottom of the barrel, the samples have more sediment.

Red crystals: the crystals come from Merlot, which was also aged in a 10,000 litre round oak barrel. The maturation process lasted for 9-12 months. Compared to Refošk, this wine forms less tartar, because it has lower acid contents and is therefore slower to mature.

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