Science Page 4
Titanium
The pic is of rutile.
Titanium has behaviour that is complex but not fully understood, so it won't all be covered in this week's post.
It is most commonly found as TiO2, but also as Ti2O3 and TiO. And TiO2 can be found in two forms: anatase and rutile. But anatase changes irreversibly into rutile from 850C, so we are mostly dealing with rutile.
Having the formula TiO2, many people think that it is be a glass former, but this is not the case. Basically, the Ti ion is about 50% larger in diameter than Si, and so it doesn't fit within the tetrahedron of oxygen atoms. The oxygen atoms are forced further away, enlarging the tetrahedron, but then the interatomic bonds are not strong enough for it to be a glass former. But, just to show that there are always surprises in life, a mixture of TiO2 and K2O does create a glass, even though neither are glass formers!
For quantities of up to about 10% molar, the Ti atoms can just replace Si in the glaze matrix. But then this becomes too distorted, and we get more complex structures - which I'll go into next week, along with the effects of titanium on phase separation and on crystal growth.
Today, I'll look at how titanium affects colour, starting with just titanium and no other colourants. Low amounts, of up to about 5%, remain dissolved in the glaze matrix as it cools, and are pretty well transparent. Above this, and anatase or rutile crystals will form. Anatase is a bright bluish white, but except for low fired glazes we always end up with rutile, which has a creamy colour.
But in the right conditions TiO2 can reduce to Ti2O3, giving violet/purple/blue colours, with just 2% molar Ti. This can happen in reduction firing, or in phase separated boron or phosphor glazes.
It can also interact with a number of colourants, changing at least a proportion from being a network modifier to a glass former, with an accompanying change in colour and raised melting point. Iron (Fe3+) amber takes on a deeper colour; copper (Cu2+) goes from blue to green or brown; uranium goes from uranyl to bright yellow uranate; and manganese goes from a weak yellow to a strong amber, or orange if cerium is also there.
Titanium - nucleation
It's well known amongst potters that the addition of titanium to a glaze can encourage crystal growth, so let's look at how this works.
The image shows the viscosity and temperature of a silica-alumina glaze with lithium as the flux (and small amounts of MgO and ZnO), undergoing heat treatment - this is how those working in glass do it, with potters getting similar effects with controlled cooling. The top is with no titanium, and the bottom is with 5% by weight of titanium dioxide added. It was found that, for this formula , more than 7% of TiO2 resulted first in phase separation and then in devitrification, and these amounts would not have produced crystallisation.
Without titanium, the viscosity of the melt is proportional to the temperature, and this (together with other tests) shows that there were no structural changes - it stayed a homogenous liquid throughout.
With titanium, during the hold at 750C the viscosity increases 10 fold, suggesting phase separation or nucleation. But then there's a marked drop in viscosity, which then climbs again; here the glaze is devitrifying, with the formation of high (or beta) quartz crystals. And the final drop and increase is the quartz and lithium reacting to form beta spodumene ( LiAl(SiO₃)₂ ), which converts to the normal alpha form on cooling, and a small amount of tieilite (Al2O3⋅TiO2).
So what is nucleation, and why is it helpful? Nucleation is the formation of small seed crystals that are large enough to be stable, and so for larger crystals to grow round them. Their formation needs a lower temperature, otherwise the melts has too much energy and the seeds are unstable, rapidly appearing and then dissolving again; and it also needs a nucleating agent (or catalyst), in this case titanium though others such as zircon or phosphor are also often used, and others also work. After nucleation, the temperature can be raised to encourage crystal growth, with one crystal forming round each seed - the crystals don't always have the same formula as the seeds.
Without nucleation, many compositions will not form crystals, and the extent of crystal growth is generally much less.
Titanium Forms in the Glaze
Carrying on with titanium, in this post we'll look at the forms it can take in the glaze.
Starting with the dioxide, TiO2, as a source material the crystals are octahedral in all the common forms. It cannot be a glass former due to the large size of the Ti atom, but it can form a tetrahedron that wedges into the silica glaze matrix. You can see this in the top image, with Si in yellow, Ti green and O red. As always, when the Ti is an integral part of the glaze like this it is in its most stable form - but the Ti-O bond is significantly weaker than the Si-O bond, so adding Ti will weaken the glaze. In some cases of very low Ti there may be phase separation, with some of the Ti forming a seperate Ti only phase.
But, because of its larger size, there's a limit to how much can fit in as standard tetrahedra. Researchers are still working on exactly what happens, but it looks like one of the Ti-O bonds becomes a double bond. This means that this O cannot connect to anything else, thereby acting as a network modifier, but to make the charges balance the Ti has to link up with an alkali. This is shown in the bottom molecule, with Na in , and a broken bond between Si and Ti. And with high Ti in some cases the Ti can form an octahedral shape, becoming more disruptive. In both these cases, the breaking up of the glaze network weakens it further than when the Ti is forming a normal tetrahedron.
Some tests in an Al free glass show that there may be phase separation up to 2% molar, then Ti tetrahedra up to 10%. Above this we starts to see double bonds, which dominate from 15% on, but the numbers vary with the glaze composition. Increasing Al significantly reduces the formation of double bonds. Alkali earths tend to promote the double bonds more than alkalis, and for both groups the smaller ions increase the formation of double bonds.
To complicate things further, you can also get flakelets of Ti and alkalis forming, with the fluxes surrounded by Ti. These actually strengthen the glaze, because they reduce the amount of flux in the main silica network, thereby strengthening it (assuming that there is enough left to bond with Al and any 
.
Titanium and Crystals
Image: Titanite crystal
Many people will know that titanium acts as a powerful nucleating agent, or seed, for crystalline glazes. Adding as little as 1% molar will promote crystal growth, and adding larger amounts often produces a larger number of smaller crystals. Yet the crystals that form do not contain titanium - unless you add enough to grow rutile (TiO2) or titanite (CaTiSiO5) crystals - titanite can be luminescent, which may be fun to explore.
Let's step back and look at what happens when crystals grow. As a starting point we have a homogenous molten glaze, where the temperature is too high and the movement of the atoms is too dynamic for crystals to start to grow, or even if a few atoms started to form a crystal it would quickly dissolve back into the melt.
Now, if we drop the temperature a bit, the energy of all the atoms is decreased, and also we get to a temperature below the melting point of the crystals we want to grow, so they will begin to precipitate out, and be stable enough to have a more permanent existence. From these seeds, they crystals will grow - and the more seeds there are, the greater the number of smaller crystals that will be formed.
In general there is one temperature at which the seeds form, relatively low to give the stability that is needed for them to grow to a sustainable size, typically up to 50C above the glass transition temperature, Tg, at which the glaze becomes set, and atoms can no longer move within the glaze.
But at this temperature the glaze is too viscous for the crystals to grow quickly, so once the seeds are large enough to be stable we can raise the temperature to a point where the crystals grow more quickly - though for some glazes the temperature difference is minimal. I haven't really worked with crystalline glazes, but many suggest 1100C is a good starting point for this faster growth.
So what does titanium do? Well there are various suggestions as to how nucleating agents work, and different ones work in different ways. For Ti, it seems to be that it forms irregularities in the molten glaze (e.g. Al2Ti2O7 and titanates), and these act as seeds. It also reduces viscosity, which speeds crystal growth.
Click here for some additional titanium glaze notes that I have yet to incorporate properly
Iron
Iron Sources
Iron sources
Iron is one of the commonest materials we deal with, and it comes into our ceramics in multiple ways, so I thought I'd start off with a look at some of the materials containing significant proportions of iron - I'll ignore those where there's just a trace of iron as an impurity.
Red iron oxide (RIO) or haematite - Fe2O3
The main source of iron for most potters. Most analyses say it is 100% pure, though some give a small LOI. Some potters differentiate between natural and artificial versions, though I've not noticed a difference. On heating it decomposes to Fe3O4 and then FeO, giving off oxygen.
Wüstite or ferrous oxide - FeO
It is not that commonly found, as it tends to form Magnetite and haematite
Magnetite - Fe3O4, which is a mixture of Fe2O3 and FeO at a molecular level. The formula is sometimes written as FeO.Fe2O3 for this reason. Other mixtures in different proportions also exist.
Iron spangles are coarse particles of Magnetite.
Iron carbonate (Siderite) - FeCO3, decomposes at around 500-600C to FeO, giving off CO2
Iron chloride may be ferrous or ferric chloride. Both are highly soluble in water, and both occur in anhydrous or hydrated forms.
Ferrous chloride (FeCl2) is mostly found as FeCl2.4H2O. The water is driven off at 105C, and then at about 530C it decomposes to a mix of Fe2O3, Fe3O4 and hydrochloric acid.
Ferric chloride (FeCl3) is mostly found as FeCl3.6H2O. This melts at about 40C, then gives off water and hydrochloric acid and forms Fe2O3 and FeO.OH in the range of 100-600C.
Iron sulfate may be ferrous or ferric. Both are readily water soluble.
Ferrous sulfate (aka copperas) mostly occurs as FeSO4.7H2O. Water is given off between 60 and 300C, then at 680C it decomposes to Fe2O3 and sulfur dioxide and trioxide.
Ferric sulfate occurs as Fe2(SO4)3.xH2O. Water is given off at low temperatures, then it decomposes at 500-700C to form Fe2O3 and SO3
Yellow iron oxide, iron (III) oxide-hydroxide, rust, or goethite and other forms - FeO(OH).xH2O
This is a hydrated iron oxide, and can have varied quantities of water attached to it. If x=1 then it is yellow iron oxide. On heating, it converts to Fe2O3.
A variety of earth pigments (effectively mostly clay) rely on iron, and often manganese, for their colour:
- Yellow and brown ochre are hydrated iron hydroxide (FeO(OH).xH2O) (limonite)
- Red and purple ochre contain Fe2O3
- Sienna has limonite and goethite (both FeO(OH).xH2O) plus up to 5% manganese oxide.
- Umber is similar to Sienna, but up to 20% manganese oxide
The prefix burnt means that the material has been calcined, so the water of hydration has been driven off, and some limonite and goethite has been converted to haematite, giving a redder colour.
Traditionally, crocus martis also fell into this category, but nowadays is more likely to be iron sulfate.
Rutile and ilmenite are mixtures of Fe2O3 and TiO2, with ilmenite having aignificantly more iron.
Granite, basalt and other rocks can contain significant amounts of iron.
And, of course, most of the clays we use contain iron, varying from the large proportions in terracotta to just traces in kaolin and other white clays.
Iron may also be found as an impurity in small amounts in many of our materials.
And then, of course, there's Irn-Bru. The advertising says it is "Made in Scotland from Girders" - but unfortunately contains barely a trace of iron!
Iron Decomposition
Last Sunday I ran through the main sources of iron. Today I'll look at how they behave when heated up in the kiln. As the less stable forms all decompose to the most stable forms, I'll start with the least stable and work down, until they're all at the most basic oxides of Fe2O3, Fe3O4 and FeO.
Iron (III) Chloride - Ferric chloride - FeCl3.xH2O
Generally found in the hydrated form, bonded to a number of water molecules, often 6.
On heating, the hydrated form melts at 35-40C, and starts decomposing at 100C, forming Fe(OH)2Cl at 250-300C, then Fe2O3 around 400C.
The anhydrous form slowly decomposes to FeOCl in air at 220C, giving off water and hydrogen chloride. This decomposes to Fe2O3 and chlorine in various stages 350-370C. Otherwise, FeCl3 melts at 308C and boils at 315, then at 370 decomposes to Fe2O3 and chlorine.
Iron (II) Chloride - Ferrous chloride - FeCl2.xH2O
The hydrated forms decompose to the anhydrous below 100C. This then melts at 674C, boils at 1023C, but before then in air it decomposes above 250C to FeOCl and gaseous FeCl3. These decomposes to Fe2O3 as for FeCl3.
Iron (III) sulfate - Ferric sulfate - Fe2(SO4)3.xH2O
Mostly found in the hydrated form. On heating, water is lost at 175C, then it decomposes at 480C to Fe2O3 and SO3 gas.
Iron (II) sulfate - Ferrous sulfate - FeSO4.xH2O
Normally found as a hydrate, at 300C the water is given off, then it decomposes to FeO, SO2 and SO3 at somewhere between 680 and 1040C(sources vary).
Iron carbonate - FeCO3
Decomposes in air - some say that at 550C it goes to FeO and CO2; others that it goes via Fe3O4 to Fe2O3 at 550C, or at 800C to FeO and Fe3O4, both of which then going to Fe2O3; and a 3rd source saying it goes to Fe3O4 at 280-490C.
In reduction, it goes to Fe3O4 below 733C, then FeO + Fe3O4 at higher temperatures, with more FeO as the temperature rises.
Limonite, goethite, yellow iron oxide etc. - FeO(OH).xH2O
On heating, the water is given off in stages from 250-5500C, and the iron is oxidised to Fe2O3 by 350C.
So next week we'll look at how the three basic oxides of iron iron behave when heated.
Iron and Ellingham Diagrams
So, continuing with iron, on heating we've got everything down to FeO, Fe3O4 or Fe2O3. But we know that above a certain temperature the red/brown Fe2O3 reduces down to black Fe3O4, and possibly FeO. The clue to how this happens is the slightly scary looking Ellingham diagram, which I'll take you through.
The red-brown lines show 4 different ways in which iron can react with oxygen.
The bottom axis of the graph is the temperature, in Celsius.
The vertical axis is something called the Gibbs free energy, which denotes how easily the reaction takes place. The higher a line is on the graph, the more easily the iron oxide is reduced towards the metal, whereas the lower it is, the more readily the metal becomes oxidised.
The oxidation or reduction also depends on the amount of oxygen in the kiln, shown by the green lines. The bright green line at the top shows the kiln in normal air, as found in an electric kiln, whereas the dark green at the bottom shows a gas or wood fired kiln in heavy reduction.
So if we start with Fe2O3 in an electric kiln, the pink line crosses the oxidation line at about 1150C, and we can expect Fe2O3 to be reduced to Fe3O4 from that temperature upwards. We can't expect any further reduction though.
Now if we switch to a gas kiln in heavy reduction, the reduction to Fe3O4 would occur at about 750C, and then at just under 1200C the Fe3O4 would further reduce to FeO.
The blue lines are reactions from C, CO and CO2, as found in reduction. Both C and CO can act as reducing agents where the blue line lies below the red line, and the bigger the distance the stronger and more rapid the reduction. So CO will readily reduce Fe2O3 to Fe3O4, and go to FeO more slowly; and carbon is a stronger reducing agent.
In practice, the graph underestimates the temperature needed, in part because we are not just dealing with iron oxide, which tends to drive the temperatures up, and in part because this is the starting temperature for the reaction, and it needs a higher temperature to happen at a significant rate. But the graphs are a useful indication of behaviour.
These diagrams cover all metal oxides, not just iron, and tell us if things will react.
Iron and Carbon Trapping
As an example of the application of last week's post, Florian Gadsby asked why his iron glazes don't do carbon trapping.
Carbon trapping happens in early reduction, before the glaze has melted. But as shown last week carbon is a strong reducing agent on iron, so the Fe2O3 is reduced to Fe3O4 or FeO. Both of these are powerful fluxes, so the glaze melts, stopping more carbon being absorbed and so stopping carbon trapping.
But this week we're looking at what happens when iron glazes cool, and why Fe3O4 and FeO don't reoxidize to Fe2O3, which would be expected. First, any reoxidization will occur on or close to the surface, as the oxygen will not penetrate the molten glaze. Secondly, I've found that glazes like temmoku and oil spots separate into 3 layers: a clear layer by the clay, then a black layer, and finally a clear layer on the surface. This surface clear layer stops the iron deoxidizing. But - there's always a but - the glaze needs to be thick enough for this separation to occur. This is why a thick temmoku is black, but a thin glaze doesn't have the clear layer, so the surface iron reoxidizes to red/brown red iron oxide.
And what about the oil spots? These crystalize out on the glaze surface, and have long been thought to be Fe3O4 or FeO. If so, they would probably revert to red iron oxide. But the latest analysis shows that they are the rarer epsilon form of Fe2O3 instead of the more common alpha form, so they stay gunmetal grey. But if the firing schedule is wrong the alpha form will develop, so the oil spots are red/brown, often larger and a bit runnier.
The image shows a large bowl with an extreme oil spot glaze I made a while ago.
Iron in the Glaze
Image is a ceramic foam made from iron rich fly ash and waste glass
So now we've got our iron-bearing components down to Fe2O3 and FeO, how do these behave in the glaze?
The first thing to note is that as the temperature increases, and as any level of reduction increases, the iron will increasingly transform to FeO in preference to Fe2O3 (and vice versa). But it isn't quite as simple as that. Fe2O3 wants to link up with a compensating alkali or alkali earth to form a tetrahedron, and be part of the silica network, just as aluminium does.
But if there aren't enough compensating atoms to go round, which may be the case as the amount of iron increases, then the balance shifts towards more FeO, both because Fe2O3 cannot do what it wants, and because the Fe2+ in FeO can link up with Al3+ or Fe3+, enabling them to form tetrahedra.
As a further complication, not all of the alkalis and alkali earths behave equally. Basically K is the best at promoting Fe2O3, and then the order descends as Na, Ba, Sr, Li, Ca and then Mg, the latter strongly promoting FeO.
Within the glaze, Fe2O3 mostly acts as a network former, but it is much weaker than Al or Si, so its presence results a lower melting point and less durable glaze. But there is also evidence that Fe3+ can act as an octahedral network modifier as well.
From the behaviour of FeO, lowering melting points and viscosity, and destabilising the glaze network, it could be taken to be a true flux. But again things are more complex, as it forms complex structures that are as yet poorly understood.
Another behaviour not fully understood is that in oxidation (but not reduction), in some glazes some of the iron will migrate to the surface and form as Fe2O3.
Iron tends to increase the tendency for crystals to form in a glaze, and through the depth of the glaze rather than just on the surface.
In boron glazes, iron decreases the boron tetrahedra within the silica network, and increases the tendence for phase separation to occur. This is despite the compensating atoms having a slightly higher preference to bond with B over Fe.
So, for such a common element, there's still a lot not fully understood about it.
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