Elementary, my dear Watson – Ca Calcium

As time goes past on our elementary adventures, I hope to ensure that at least one element is included from each of the eighteen Groups of the Periodic Table (a.k.a. the Periodic Table of the Elements).

So far, we’ve explored two very different elements from Group 1: Hydrogen, and Sodium. We’ve had one from Group 8: Iron, another from Group 11: Copper, plus an element from Group 12: Mercury.

We’ve seen one from Group 14: Carbon, have been a little greedy with two chosen from Group 16: Oxygen, and, most recently, Sulphur. In addition, we’ve investigated an element from Group 17: Bromine.

This time, we’re going to take a look at one of the elements from Group 2, the alkaline earth metals. This is the story of calcium.

When you think about calcium as an element, I would bet that a metal doesn’t immediately spring to mind. This is almost certainly because we rarely see it in its raw, native, state—indeed, this doesn’t occur in nature. Being a very reactive element, calcium inevitably occurs on Earth in compound form. Thus, we tend to associate it with things like chalk, or bone, oh, or those White Cliffs above.

In fact, elemental calcium is a silvery-white shiny metal, with just a hint of a slight yellowish tint. Solid at room temperature, it tarnishes very rapidly in air, forming a dull, dark grey oxide-nitride layer which obscures its metallic appearance. Below, it is seen in its elemental form under argon gas (an inert noble gas with which it cannot react), so calcium’s metallic nature is instantly recognisable.

Calcium is a pretty commonplace element. It’s the fifth most abundant element in the Earth’s crust, where it occurs most commonly as rock formed from the fossilised remnants of early sea life: limestone (calcium carbonate), gypsum (calcium sulphate), fluorite (calcium fluoride) and apatite (calcium chloro- or fluoro-phosphate).

Somewhat intriguingly, given this ‘origin’ on Earth, substantial deposits of calcium have been identified on the Moon. Actually, that’s a little tongue in cheek. In truth, it’s thought that there is a stellar nucleosynthesis origin for pretty much all known elements. Calcium was likely produced by the explosion of massive stars (supernovae) and white dwarfs. The sea life incorporated what was already present on Earth.

As you read this article, you are 100% ‘star stuff’, and about 1.5% of what makes you ‘you’ is calcium. Of that, almost all of the calcium that can be found in your body (in fact, close to 99%) is in your teeth and bones. It does a lot more in the body than just sit there though, and we’ll come to that later.

Like some of the soft Group 1 metals, calcium can be cut with a knife, though it takes considerable welly to do this. It crystallises with a face-centred cubic structure and has no known allotropes.

It does, however, have a number of isotopes, twenty-six in fact, five of which are stable. These are ⁴⁰Ca (the most abundant, comprising nearly 97% of the calcium found on Earth), plus ⁴²Ca, ⁴³Ca, ⁴⁴Ca, and ⁴⁶Ca. Calcium also has a variety of radioisotopes, some twenty or so, from ³⁵Ca to ⁶⁰Ca.

Of these, ⁴⁸Ca, a scarce isotope with 20 protons and 28 neutrons, has such an incredibly long half-life (at 6.4×10¹⁹ years, way more than I can even contemplate) that it can pretty much be considered stable. It’s an interesting isotope though. Because it is more or less stable and it has a lot of neutrons, it is a useful starter material in the hunt for ‘new’ synthetic elements, which are created in nuclear reactions (usually by nuclear fusion). The five heaviest known elements on the Periodic Table, flerovium (F), moscovium (Mc), livermorium (Lv), tennessine (Ts), and Oganesson (Og) which we have met in a previous article, were all synthesised using ⁴⁸Ca as a ‘seed’ nucleus.

There is also a practical application for isotopic calcium in the form of potassium–calcium dating (K–Ca dating). This technique is used to determine the age of fossils, rocks, and sediments, but it’s fraught with complications. Although no other use for calcium’s other isotopes has yet been identified, it may have potential within medicine, since calcium is such an important element in biological molecules. Also, looking at recent geochemical and biogeochemical research, calcium isotopes seem to offer some promise there too.

As we would expect for metals, calcium and the other Group 2 elements conduct heat and electricity pretty easily. Interestingly, although calcium doesn’t conduct electricity as effectively as copper or aluminium, it is under consideration for use as a conductor for various applications in space programmes. This is because, when considered by volume, it is a very low-density material (always a handy attribute when you are trying to lift mass off planet Earth). Additionally, in space, it would remain stable in metallic form, since space is devoid of the oxygen-rich terrestrial atmosphere with which calcium so readily reacts.

As with the Group 1 elements (the alkali metals), most of the Group 2 elements burn to give a flame of a characteristic colour. This provides a relatively straightforward way in which individual elements can be identified.

We spoke about the colours for Group 1 metals, when we looked at sodium, but for the Group 2 elements, beryllium (Be) gives us a white flame, though it is quite reluctant to burn. Magnesium (Mg) burns to give the characteristic strikingly intense brilliant white flame you might recall from school days. These two elements burn without exhibiting a discernible colour since the wavelengths of the energy emitted fall outside of the visible spectrum.

Moving down the group, our element, calcium (Ca), burns with a robust orangey or brick red flame. Strontium (Sr) displays a more crimson hue, and barium (Ba)has an apple-green flame. The colour which might be produced by radium (Ra) has been guessed but has never actually been observed.

As before, the colour of the light emitted depends on the energy required to move electrons from one orbital to another, and the colour emitted by larger atoms is lower in energy than the light emitted by smaller atoms.

The most important difference between the Group 1 alkali metals (like sodium, which we have already considered) and Group 2 alkaline earth metals (like calcium) is that the Group 1 alkali metals have just a single electron in their outermost orbit (electron shell) whereas Group 2 alkaline earth metals have two electrons in their outermost orbit.

Because of the ease with which these outer electrons can be donated, all of the metals in both of these two groups are highly reactive. That said, the Group 2 elements are slightly less so than those in Group 1, the alkali metals. Our element, like all Group 2 metals reacts readily with air, but also with water (well, as always, there are exceptions, and beryllium, the Group’s oddball, doesn’t react with water). Group 2 metals also react with acids, typically evolving hydrogen gas.

But not only are there striking similarities between these two groups but, leaving aside the number of valence electrons, there are a few other differences between the alkali metals and the alkaline earth metals too.

Coming back to the element we are concentrating on today, calcium, what can we say about it?

Well, its compounds were certainly well known in antiquity, and appeared prominently in ancient technologies. Lime (calcium oxide, CaO), for example, was a particularly useful material that had for a very long time formed the basis of mortars and plasters.

However, as an element, calcium was first isolated in 1808 by our old friend Sir Humphry Davy. Davy named this element too, taking as a root the Latin word for lime, calx.

Davy had acted upon a suspicion that lime well might be the oxide of an unknown element, as the French chemist Antoine-Laurent Lavoisier (another old friend) had posited. As Lavoisier had been unable to break down this compound, lime, into its components, he’d eventually classified it as an ‘earth’ (in this designation we begin to see where the idea for the name of Group 2’s elements, the alkaline ‘earth’ metals, comes from).

Unsurprisingly, as he was a very methodical man, Davy strove to make use of the ‘voltaic pile’ developed by Alessandro Volta, using the same technique which led to him isolating sodium. His first attempt was to try to reduce moistened lime by electrolysis. However, although he had successfully isolated both sodium and potassium in this manner, calcium is somewhat recalcitrant, so his experiment failed.

Davy’s follow up experiments were undertaken in several stages. Thankfully, he had received a letter from the Swedish chemist Jöns Jacob Berzelius (another gentleman we’ve encountered in previous articles). This stated that Berzelius had, in collaboration with the Swedish king’s physician, Magnus Martin Pontin, taken a vital step which would help him in his quest.

They had, they claimed, successfully decomposed lime by mixing the compound with mercury and using a mercury cathode to electrolyse it. This allowed a calcium-mercury amalgam to form (remember those, from the mercury article?).

Davy tried this process himself and, OK, there was indeed some amalgam created. But there wasn’t really enough of this stuff to do anything with it, so Davy tried again. This time, he chose to use more lime in the mixture. On this occasion he produced enough of the amalgam to allow him to distil off the mercury (which has a very low boiling point). At last, this left him with a sample of pure calcium. Success! He later went on to use this modified method to successfully isolate strontium, barium, and magnesium.

We’ve already seen that the lime Davy used was a substance with a considerable history. Indeed, the Romans well understood that limestone (a carbonate-based sedimentary rock formed in shallow marine environments, primarily composed of calcium carbonate, CaCO₃) can be heated, typically in a kiln, to above 700 °C to produce the highly caustic ‘quicklime’. This substance, a.k.a. burnt lime, is calcium oxide (CaO).

Quicklime is formed in a procedure called ‘lime-burning’ or more commonly ‘calcination’. In this reaction, carbon dioxide (CO₂) is lost as a gas. Quicklime isn’t a stable material though, and as it cools it will gradually react with CO₂ in the air, slowly turning back into calcium carbonate.

However, if mixed with water, this progress can be halted as the quicklime reacts with the water, forming what’s known as ‘slaked lime’. Chemically, this is calcium hydroxide, Ca(OH)₂. This material has long been used as the basis of excellent lime plaster, and lime mortar. At this point other materials such as ash, bone, dung, plant matter, sand, or volcanic debris, can be added to the slaked lime to alter the characteristics of the end product. Alternatively, if a greater quantity of water is added to the mix, the resulting slurry becomes limewash.

As each of these moist materials (plaster, mortar, or wash) slowly ‘dries’ or, more accurately, chemically ‘sets’ (note that time is important, it cannot be hurried), it absorbs carbon dioxide from the air since this gas is water soluble. This stage of the process is called ‘carbonation, and the material gradually becomes calcium carbonate again, with water being lost to evaporation as part of this reaction.

In its entirety, this is a circular process, both starting and ending with calcium carbonate, CaCO_3. It’s better known as the ‘Lime Cycle’.

Quite amazingly, this technology was evidently understood, at least empirically, and used by our ancestors well before the Romans ever took an interest. In fact, knowledge of how to manipulate lime, and the skills to do so goes as far back as the Neolithic.

In Israel, a cemetery site has been excavated within the past few years at Nahal Ein-Gev II (NEG II), in the Upper Jordan Valley. This has been radiocarbon dated to the 10^th millennium BC. As the excavations progressed, the remains of some eight individuals were observed to be covered, or shielded, by a layer (about 16 inches thick) of a white dense material.

Analysis confirmed that this was, as had been suggested by the site archaeologists, pyrogenic lime plaster, which is formed when limestone (or marl, or chalk) is heated near to or above 700 °C, then mixed with water to make a malleable putty-like plaster. This important find pushes back by some two thousand years the date which had been previously believed for the first deliberate use of calcination, slaking, and carbonation.

Once established, the technology spread, and material evidence shows that Pre-Pottery Neolithic peoples of the Levant made good use of it in the 8^th millennium BC, not least in their works of art. The extraordinary reed and plaster busts and statues of ʿAin Ghazal found in Jordan evidence this. Lime mortar was also known to be in use by around 6,500 BC in the pre- and early-Harappan cultures in the Indus valley region.

By around 4,000 BC, the Egyptians were using both lime plasters and lime mortars, both of which ‘harden with age’, in the construction of the pyramids. In many cases, several different types of plaster were used. Lime plaster was often chosen for its durability, being laid down as a ‘base coat’ (this comprised lime and sand, reinforced with animal hair, as a ‘stucco’ laid onto a substrate of reeds laced together with cord). The finest, outer layer might then be of a fine lime plaster or sometimes of gypsum plaster (another calcium compound, in fact one we know as ‘Plaster of Paris’). Gypsum plaster, whilst smooth, was less durable. I would suggest this demonstrates a clear indication that the architects and builders knew their materials through and through and were already well-versed in selecting the appropriate substance for the task at hand.

A variety of different mortars were also used in constructing the pyramids, again with different compositions presumably selected by the Egyptians based on their specific characteristics and the purpose they were intended to serve. It is clear they knew a great deal about lime mortars too. Although not commonplace, the structural mortars used to join the huge limestone core blocks of the Pyramid of Menkaure in Giza (constructed around 2,510 BC) are lime-based mortar. Interestingly, despite all being constructed at the same geographical location, and at roughly the same period, for the larger, and slightly later, Pyramid of Khafre (built around 2,570 BC) and the Great Pyramid of Giza (a.k.a. the Pyramid of Cheops or Khufu, erected around 2,560-80 BC), gypsum mortars were used.

Moving onwards to the Romans, with whom we often associate the use of lime. They experimented with a whole raft of lime-based building materials, and devised a number of construction compounds which really did stand the test of time. Not only did they modify the, by now familiar, non-hydraulic lime-based materials (the type which set by carbonation—see above) but, by the 2^nd century BC, they had developed a completely new material. This was one which would set and become harder as it aged in a wet environment, even underwater. This mortar was a hydraulic setting lime material, which they used in combination with an aggregate of some sort, and called opus caementicium, otherwise known as Roman concrete.

Roman concrete was made using slaked lime as a binder, but instead of relying on carbonation to set, the addition of some other crucial components changed matters chemically. The new ingredient, which reacted with the lime in the presence of water to form the hydraulic variant was pozzolana, or the sandy or dust-like debris or ash from volcanic eruptions.

Pozzolana typically contains high levels of both silica (silicon dioxide, SiO₂) and alumina (aluminium oxide, Al₂O₃). In what is called the ‘pozzolanic reaction’, these minerals react with the lime in the presence of water to produce a building material which was strongly cohesive, and eminently practical, particularly as it could be moulded to the required shape. In fact, the material was better than this. Because of the way it was produced, a hot-mixing process, curing and setting times were reduced, so construction could be carried out much more quickly.

But one further secret made this material truly extraordinary, and the facts of this have only recently come to light. In all Roman concrete, it is possible to see ‘lime clasts’ in the mixture (small white lumps or flecks which remain in the concrete long after it has set). These were long thought a fault, a result of poor mixing as if the makers were rushing the process.

Not so, and researchers at Massachusetts Institute of Technology have recently been able to recreate the exceptionally durable materials the Romans perfected. Careful analysis showed that the Romans had not only included slaked lime in their recipe, but quicklime too. The chemistry which takes place because of these clasts allows Roman concrete to be a ‘self-healing’ material. When water seeps into cracks, it encounters the lime clasts, which chemically react with the moisture and atmospheric carbon dioxide to lay down new material, in the form of calcium compounds, to close the cracks.

For the Romans, they were no longer constrained to building structures with a stone block or brick-built core, held in place with mortar. Now, the new material allowed for new styles of architecture which were formerly challenging or impossible. Roman hydraulic concrete was a complete gamechanger, utterly revolutionising Roman construction techniques and architecture. In fact, the structures erected because of its development and use became known as the Roman architectural revolution.

At first, it was used to build harbour walls, including those parts underwater, and a number of other structures around Baiae in the Gulf of Naples. Later volcanic activity meant that some of this area was inundated by the sea, but the buildings, made from this new Roman super-material, can still be seen more than two millennia after they were built, by visiting both land and underwater archaeological parks.

Other examples can be seen at Caesarea, in what is now Israel, in the traces of what remains of once massive breakwaters constructed around the existing small harbour to create vast middle and outer harbours. These were commissioned by King Herod around 22 BC in what was then the capital of Roman Judaea. They were a quite remarkable undertaking, the sheer scale of which can be visualised from a reconstruction HERE. Contrast the scale of the breakwaters, bearing in mind these were built down to considerable depths, with the theatre, which comfortably seated some four thousand people.

Roman concrete wasn’t just used for aqueducts, baths, and marine structures though. The architects soon realised that without some of the old constraints, a rich new monumental style was feasible. Vaulted ceilings, arches, domes, buttresses, decorative columns which no longer purely served a supporting role, were all incorporated into new buildings.

Indeed, Rome’s Pantheon, which was built as a temple to all of the gods—hedging one’s bets if ever I heard it (and, incidentally, the Italian site most often visited), has a beautifully designed coffered concrete dome. This is a veritable masterpiece of civil engineering. The dome, with its iconic ‘oculus’ (massive, at 25 feet 7 ³/₃₂ inches in diameter) is part of a structure commissioned by the Emperor Hadrian, yes, he of ‘the wall’.

The Pantheon’s dome was the largest in the world for over a thousand years, and that indented ‘stepped’ design is actually eminently practical as well as exquisitely striking as it reduces the dome’s weight the higher up towards the oculus it gets. Even today, the Pantheon’s dome remains the biggest unreinforced concrete dome in the world, having survived earthquakes, floods, wars, and a steady cavalcade of tourists for nearly two millennia!

Concrete wasn’t exactly the prettiest material though (and aesthetics were immensely important to the Romans), so for many structures it was often used to form a core, then faced with more attractive tile or stone, often the finest marble. This in itself is a fascinating architectural progression. Perhaps someone with a more in-depth knowledge of construction techniques might oblige with another article?

We make much of this momentous Roman technological step, and they do indeed deserve a great deal of credit. They certainly took the idea and ran with it. However, on a much smaller scale, some five hundred years before them, the Nabataeans, an ancient Semitic people from northern Arabia and the southern Levant had, in fact, already both discovered, and put into use, the secrets of hydraulic lime. The Nabataeans used their ‘concrete’ to construct the positively hush-hush underground waterproof cisterns which allowed them gather and conserve every drop of precious rainfall, using covered channels (often hidden), dams, and reservoirs to harvest the water which fell during flash floods in the winter season. This allowed them to flourish in the harsh desert environment they inhabited, providing a reliable source of drinking water for both the people and their flocks in sophisticated cities like Petra.

Sadly, with the fall of the Roman Empire, the techniques for making hydraulic, pozzolan or Roman cement were lost. It took a thousand years before ‘concrete’ became a material of interest again. Even then, it took centuries, in fact until the 1750s, before the English civil engineer John Smeaton (definitely a man worthy of an article in his own right), rediscovered and perfected the use of hydraulic lime. This he used to construct the third, and most iconic, lighthouse on the Eddystone Rocks after the previous two had already been destroyed in violently tempestuous conditions.

Astonishingly, although the lighthouse has now been replaced by a fourth (Douglass’s lighthouse, completed in 1882), Smeaton’s foundations and the lower section of his lighthouse remain on Eddystone Rocks to this day. Having weathered countless storms, unremittingly battered by the elements, they were simply too strong to be dismantled when the modern lighthouse was built. Once again, quite a testament to an incredibly tough material.

In the meanwhile, the recipes, and technologies of non-hydraulic lime-based construction materials like plaster, wash, and mortar (with lime acting as the ‘cement’ or ‘binder’) were not lost, so continued in use to a quite staggering degree. This remained the case until they too were finally overtaken by ‘modern’ materials, such as the one Joseph Aspdin perfected and patented as his ‘Portland cement’ in 1824.

Interestingly, traditional building materials such as lime plaster, paints, putty, mortar, limewash, and more have seen a huge renaissance in the last few decades. At the forefront of this stands a British couple, Nigel, and Joyce Gervis. They’d bought a ramshackle old farmhouse in the Brecon Beacons, in the early 1990s, with a view to its sympathetic restoration as a family home. But they immediately hit a major problem, in that they simply could not source the lime-based materials they needed and, worse, much of the knowledge around this technology appeared to be pretty much lost.

Undaunted, they figured out how to recreate these materials themselves. Furthermore, they went on to establish an extremely successful business, named after their farmhouse, Tŷ-Mawr, to sell their products far and wide, running specialist courses alongside this to keep these traditional building techniques alive.

Incidental to the resurgence in the use of such traditional materials, because of the way in which they set, they have recently piqued considerable interest for another reason. They are being seen as providing the potential means to reduce CO₂ levels in an increasingly ‘zero-carbon’ focused world. Make of that what you will.

Before I move on to other uses of our element, calcium, I just want to say something about the part that lime (calcium oxide, CaO and, more usually, calcium hydroxide, Ca(OH)₂) plays in things like mortar and concrete. To be quite frank, such terms utterly bamboozled me, and I had thought they, and cement, were pretty interchangeable, but it turns out I was way off base.

Both mortar and concrete have additional materials added to the mix to modify its properties (usually making it more durable, or more rapidly setting), but the lime is what holds it all together as a binder. In the example below, it’s the ‘cement’, although we use that word in a slightly different sense in today’s world.

OK, we’ve got an important aspect of calcium’s usefulness out of the way, so let’s look at a few other ways in which our element is used.

I‘m aware that I keep banging on about elements which are essential for living organisms, both plants and animals, and in this article I won’t repeat the modified version of the Periodic Table which shows which elements are essential. Calcium, though, is no exception. As we humans require a notable amount of calcium, typically at levels higher than 100 mg/day, it is classed as one of the macrominerals essential for human life.

As a reminder, macrominerals are needed by our bodies so that its basic functions can be sustained. They are important in a wide variety of metabolic (life-sustaining) and physiological (proper function) processes, ensure that our organs work as expected, and our bodies grow and develop as they should.

In particular, calcium (along with phosphorous) is crucial to the contraction of muscles and to the function of our nerves, indeed it is central to the process in which the light which hits the retina in our eyes is transformed into electrical impulses and transmitted to be interpreted by the brain.

In addition, i is fundamental to maintaining the appropriate levels of mineralisation in our skeletal tissues. Like most bodily tissues, old bone is constantly being removed from the skeleton (bone resorption) and new bone tissue formed (ossification). This is a continual, endless process which takes place throughout our lives and is known as bone remodelling. This is how, if you break a bone, it can heal, remodelling can repair the bone to be as strong as it was before the break.

We’ve already observed that almost 99% of all of the calcium in our body, in the form of calcium hydroxyapatite, Ca₁₀(PO₄)6(OH)₂, is located in our bones and teeth. Put bluntly, without calcium to build and maintain the skeleton which supports us, we’d be a toothless bag of soggy goo.

Calcium also plays an important part in blood clotting, regulating heart rhythms, and enzyme function. It is one of the most abundant electrolytes in our bodies. It is vital to the function of our cell membranes (in which there are dedicated calcium ‘channels’ to transport calcium ions across the membrane, in and out of the cell), and aids in cell signalling, allowing cells to receive, process, and transmit signals backwards and forwards with its environment and internally.

But where does this calcium we need come from; you might ask. Well, the body can draw calcium from our bones, but ‘borrowing’ in this way is neither a sensible nor sustainable long-term measure. So, we need to take in calcium from our diet, and there are a number of foods which are particularly rich in this element.

Dairy products top the list (cheese, milk, cream, yoghurt, butter, etc.), but fish (especially tinned fish with bones, like sardines, anchovies, and salmon) is a good source, as is tofu, dried figs, seeds, and nuts (and their butters), lentils and some beans, and many leafy green vegetables (including broccoli, watercress, spinach, kale, etc.).

Us ‘more mature’ types, whether we’re male or female, require about 1200 mg of the stuff in our diets every day to remain in peak health. A deficiency can lead to osteoporosis, something which, when I was younger, was called ‘brittle bones’. Just a tip, but a rather important one: don’t forget, we also need Vitamin D to best absorb calcium, so get yourself out in that summer sun while we have it.

In cattle, as for all milk producing mammals, calcium is essential for optimum milk production. This is important in dairying, but also directly affects the growth rate of an animal’s offspring. Interestingly, the first milk an animal (including humans) produces is colostrum. This plays a central role in developing the newborn’s immune system, and contains around twice as much calcium as the milk produced thereafter.

Birds, including our domestic fowl, need calcium in order to produce their eggs, since the shells are almost entirely of calcium carbonate. Oh, then there are the animals with shells and exoskeletons. Whether they be snails, scorpions, or scallops, they too need calcium. For them, it is absolutely central to keeping their ‘mobile homes’ up to scratch.

Looking at plants, just as we need calcium to maintain our structural components a.k.a. our bones, so plants require calcium (as calcium pectate) to build and maintain strong cell walls, helping make them grow tall and strong. Helpfully, calcium also binds to other nutrients that plants need, and thus aids in their transport around the plant. Vegetation can take up calcium through the roots, from the soil (which is where bone meal fertilisers, or leaving finely ground eggshells at the foot of your plants comes in), but they can also absorb it through the leaves (this is why foliar feeding can be helpful).

OK, this is all good stuff, but as Brother Reg from ‘Monty Python’s Life Of Brian’ might have said, apart from the building materials and our health, what has calcium ever done for us, eh?

Well, there isn’t a massive demand for elemental calcium, because of its highly reactive nature. That said, it is used to refine thorium (Th) and uranium (U), reducing them to a metallic form by removing impurities. The processes are multi-step, and quite complex, but for thorium, this uses the Spedding process, and for uranium a modified version of the Ames process. These entail Th or U being heated to extreme temperatures in the presence of calcium metal, which reduces them, stripping away the unwanted elements to leave a pure metal. These processes were developed in the 1940s as part of the Manhattan Project that produced the first nuclear weapons.

In a similar application, calcium can be used to remove impurities such as carbon, oxygen, and sulphur from certain metal alloys. Additionally, calcium is sometimes deliberately included in alloys to modify their characteristics, including alloys of copper (Cu), and lead (Pb), which has a considerable history. In fact, the grids in lead–acid batteries are actually made from a lead–calcium alloy.  It can, in a more recent development, also be alloyed with lighter metals such as aluminium (Al), beryllium (Be), and magnesium (Mg).

Calcium is also used as a ‘modifier’ in cast irons and steels, to modify the levels of sulphides in the liquid metal prior to casting, but also to control the shape of any remaining sulphide inclusions. Although it is possible to introduce calcium as pure Ca, this is a difficult and dangerous process. More commonly it is added in powdered form as calcium monosilicide (CaSi), or as ‘casiba’ a calcium-silicon-barium alloy (CaSiBa). The most efficient method, however, is to introduce it in wire form (where CaSi is cased in a steel sheath, also called cored wire). Since the ‘calcium treatment’ also deoxidises the steel, it is crucial that the metal be prevented from reoxidation after the Ca has been added.

This affinity with ‘impurities’ (other elements), means that calcium also finds a valuable application as a ‘getter’ in vacuum tubes and cathode ray tubes. By capturing these ‘impurities’ (e.g. traces of reactive gases, such as oxygen), the getter helps maintain the vacuum. Where do the impurities come from, to pose a problem? Well, they are absorbed into the inner surfaces of the tube’s glass sheath and can be slowly released as gases long after the vacuum has been created.

The getter can usually be seen as a silvery metallic shiny spot or a mirror-like ‘cap’ on the inside of the head of the tube. I must admit I’d often wondered why that was there.

We’ve already seen that, once reacted with other elements, and therefore in compound form, calcium has a number of uses.

You might be aware that the first stage lighting, used in music halls and theatres, was limelight (a.k.a. Drummond lights, or calcium light). This was developed in the 1820s, and credit usually goes to the Scottish army officer, and civil engineer, Thomas Drummond. However, it was actually a Cornishman, by the magnificent name of Goldsworthy Gurney. It was Gurney who directed an intensely hot oxy-hydrogen blowpipe flame at a cylindrical block of quicklime (calcium oxide, CaO), which produced a brilliant white light. Our expression, to be ‘in the limelight’, comes from this invention, since the front and centre of a stage was most well-lit, thus most attention would be focused on this spot.

Another compound we’ve already looked at, calcium carbonate, CaCO₃, is used in toothpaste, cleaning powders, and in antacid tablets. It is also used in classroom chalk, and in the manufacture of white paints, not a massive surprise as it has been used as limewash for millennia. An interesting new development though is that it is finding an application in passive cooling technology (in fact, as what are called ultra-white paints). These are highly reflective, so can help keep buildings cooler, minimising the need for air conditioning. There’s an interesting paper about the technology behind this innovation HERE.

In the form of calcium sulphate, or gypsum (CaSO₄·2H₂O), calcium has long been used in dentistry, but also (because it is biocompatible) in medicine as a means to assist bone regeneration. It’s still commonly used in construction, being the ‘rock’ in ‘sheetrock’, a.k.a. drywall, or plasterboard. Interestingly, gypsum’s name was spærstān (meaning ‘spear stone’) in Old English. This probably comes from the beautiful long pointed crystals it can sometimes be seen as.

Any of you who have had the misfortune to break a major bone will also know it as the Plaster of Paris splinting material used in those itchy and bulky white casts. It’s still in use, though a variety of lightweight, breathable synthetic casts are making headway in this area.

Speaking of casts, I certainly played around with plaster casting small statues in floppy rubber moulds as a nipper. Perhaps unsurprisingly, this technique is still used by artists in similar manner (just with a lot more skill than I managed to exhibit).

This same material (left unground) is also known as alabaster, a soft rock which has long been used for beautifully detailed carving. There was a well-regarded alabaster sculpture industry in and around Nottingham, mostly religious in nature, with altarpieces, and tomb monuments (including full-sized effigies) produced from the 14^th to early 16^th centuries.

This was Nottingham alabaster, a substance which was hugely popular, not only in Britain but across Europe. Few pieces survive completely intact here, after the brutalities of King Henry VIII’s English Reformation. But some on the continent, particularly in France, still show the bright paints which were often applied to decorate them even more richly.

The Nottingham alabaster industry managed to survive the Reformation, and continued, albeit on a smaller scale and with a slightly revised set of patterns, until the late 18th century. For those of you who’d be interested to see more of these magnificent works, the newly refurbished Nottingham Castle Museum and the Victoria and Albert Museum have decent collections.

And, finally, calcium plays a role in a process which is very dear to my heart. It’s a crucial element in cheesemaking.

Typically, calcium chloride (CaCl₂), calcium lactate (C₆H₁₀CaO₆), and calcium sulphate (CaSO₄), are used in cheesemaking, and each will have a different effect on the end product. In particular, the addition of calcium chloride can help significantly improve the cheese yield.

In the first step, bacteria are added to milk to acidify, or ‘sour’ it. The temperature is raised slightly, and the bacteria get to work converting the milk’s lactose (milk sugars) to lactic acid. As this takes place, the milk’s pH is lowered from around pH 6.5 to 6.7, to about pH 4.0 to 5.0. This begins the separation of the curds, the cheese solids, and the liquid whey.

The presence of this specially selected bacterial culture also helps control the growth of undesirable bacteria which might ruin the end product. In fact, this is the case during both cheesemaking and in storage.

Then, calcium is added, and its ions (Ca²⁺) help to bind together casein (milk protein) molecules, assisting them to clump and coagulate more rapidly, to separate them from the whey. Coagulation is the process of forming the curds.

This, in turn, determines both the texture and structure of the cheese. High levels of calcium will form the harder, and more ‘rubbery’ cheeses (such as Gruyere, Edam, Gouda, and, of course, Cheddar). Lower levels of calcium are used in the making of the softer, crumbly cheeses (like Camembert, Roquefort, Brie, and Ricotta).

But calcium, along with specific bacterial cultures (and, for blue cheeses, mould cultures), plays its part in imparting the distinctive cheese flavours too. It does this because it also affects the aging process, mostly by helping to regulate the pH level. As cheese ages and ripens, bacteria break down the curd proteins, subtly altering its flavour. This helps provide some aged cheeses, e.g. Cheddar and Parmesan, with the sharper, tangy flavour we so enjoy.

Cheesemaking is an ancient biotechnology. Although most cheeses we enjoy come from cow’s milk, the first cheeses were likely made from the milk of sheep or goats, though cattle (descended from wild aurochs, Bos primigenius) were domesticated soon afterwards.

The earliest documentary evidence for cheesemaking comes from the Neo-Sumerian peoples, who lived in ancient Mesopotamia around 2,112 to 2,004 BC. This is in the form of cuneiform administrative and accounting records, which describe the cheeses as sour and salty. Around the same time, murals which depict cheesemaking, dating to approximately 2,000 BC, were painted in Egyptian tombs.

Even before this, the ancient Greeks had a god (albeit a minor one), Aristaeus, the son of the huntress Cyrene and Apollo who was the god of cheesemaking, amongst a wide range of rustic and rural crafts. He was venerated around the same time as alabaster jars containing cheese were placed in tombs at Saqqara. These date to the First Dynasty of Egypt, about 3,000 BC.

However, the first archaeological material evidence of cheesemaking dates considerably further back. This comes from a number of sites in Poland (including Brześć Kujawski, Miechowice, Smólsk, Wolica Nowa, Stare Nakonowo and Ludwinowo). Found in the form of fragments of pierced ceramic ‘strainer’ type vessels dating to c. 5,400 BC, these were excavated from settlements occupied by the first farmers in this part of Europe.

Researchers from Professor Richard Evershed’s team at the Organic Geochemistry Unit, University of Bristol, used lipid biomarker and stable isotope analysis (employing techniques such as gas chromatography and carbon-isotope ratios) a decade ago to examine preserved fatty acids trapped within the fabric of the potsherds. Their analyses unequivocally demonstrated that the strainer-like ceramics had been used to process dairy products.

Even then, it is likely that cheesemaking well predates even this. It may have been a serendipitous discovery when pre-ceramic nomadic herdsmen stored their animal’s milk in bags made from the stomachs of sheep and goats. The stomach linings naturally contain lactic acid, rennet, and bacteria, meaning the milk very likely soured and coagulated.

With this being agitated when the bags were carried, it’s very likely a form of rudimentary cheese was produced. This could be pressed to reduce the moisture content, and salted to further help preserve it so it could be kept for later consumption. If true (it’s likely, but currently impossible to verify), this pushes the date for cheese back to around the same time as the domestication of animals, most likely in the Middle East’s ‘Fertile Crescent’ in the Neolithic period, some ten to eleven thousand years ago.

Whenever the true origins of this culinary delight, the inclusion of calcium in its making does allow me to provide you with a suitably cheesy ending. I give you Monty Python, and ‘The Life Of Brian’ (again)…

 

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