Updated: January 5, 2014
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Note: This is a "personal experience" guide, based on my own mixing throughout my time working my way up to level 140 alchemy. Your experiences may vary, and as well things change over time. Nothing here is set in stone, so use it, but use it with thoughts of your own actual play and experience in mind.)
What's useful to make? What not? In what quantities? And why?
In order, from lowest rec level to highest:
Bone Powder - Never do these just to level. At 1xp each regardless of rationality or god status, not worth it.
Mix only if you absolutely need them yourself. There are a few people who need them in large amounts, mixing Point Defense in engineering school on engineering days, but they'll likely have their own. You'll be better off just selling the bones themselves.
Fire Essence - One of the two most popular and needed essences. An absolute necessity in huge quantities, particularly for bar making. These will sell quickly and in huge amounts. As well, you'll likely need huge amounts yourself, at some point if not already there.
You'll need these in the hundreds of thousands if you're planning to get anywhere in the alchemy skill. Eventually you'll get to the point where you can afford to just buy these.
Water Essence - Not as heavily in demand as FEs, as most who need these make them themselves to save cost. But still, they do sell pretty regularly and in large amounts, though you may have to spend longer advertising them than FEs or HEs. As well, get a nice stock up for yourself if you plan to do any crafting or tailoring, as they are heavily needed to level in either skill.
Of late there has been a bit higher demand, as high level crafters have begun mixing Blank Paper in school on school day, each of which takes 5 WEs.
Earth Essence - Though there's been a slight increase in demand for seridium bars, there's still not enough demand for these to sell. Only very rarely will you find people asking to buy these. Best only to make for yourself, as needed.
The slight need for them on market comes primarily from very high level engineers who can afford to toss out Treasure Finders in school on engineering days.
Air Essence - Huge demand thanks to people who use mana draining to advance their magic level. A pain to make though, mostly due to ings (diamond and silver). For that reason, a lot of people buy them rather than make them themselves. You can get a decent price for them, and they'll sell pretty quick (though not as fast as HEs and FEs). Just expect a lot of work in the process harving the ings. Buy silver when you can find it, but expect to harvest the flowers and diamonds yourself as they're rarely if ever seen on market.
Spirit Essence - Don't make these. At all. If you have some spare mercury, make a few for yourself maybe if absolutely necessary. The reason being that the magic shop NPCs sell these for 9gc each, which is below the ing cost of self-making them. That combined with the very low (in comparison to other essies) need for them makes them pretty useless to make. Best to buy them from the magic shop as needed.
Matter Essence - They'll sell, but the price is limited by the NPC price of 12gc. If you buy all the ings, you'll break even only if you don't critical fail, which is impossible even at my level.
Harving the ings makes it one of the slowest essences to get ings for. Personally I'd recommend only making them for yourself, but if you're willing to put in the harv time, they will sell.
Energy Essence - Once you get to the point that you're using magic for portal room visits, or using point defense to level engineering, you'll be needing a LOT of these for yourself. Having a nice stock for either of those cases is a very good thing.
As for selling, they tend to do so, and at a fairly good price, so there's a pretty decent market for them. Again, they won't sell as fast as FEs or HEs so you may need to advertise longer, but a sale will eventually come. These are worth a try.
Life Essence - Summoners, especially those who are making summoning stones which require a lot of these, will buy huge amounts of these like candy if the price is fair.
As well, they're not hard to make, which makes them good for alch leveling.
Death Essence - Very rare sales, and the cost of mercury makes them pricey. Best only made for yourself as needed. Almost never seen being asked for on market.
If you do manage to sell these, you can make a small profit if you buy the mercury from Harvy in DP, and harvest the other ings yourself. However, sales are very slow on this item.
Health Essence - Always sell well, always needed, and extremely huge amounts aren't uncommon. Heavy making of these is never a bad thing as long as you don't try attempts to overprice them. There's plenty of people who can make them, and if you make attempts to overprice them on market you may find there's people who will end up never buying anything from you on principle.
Also keep in mind when pricing that they can be easily bought from NPC at 9gc each.
Magic Essence - The most-made essence that has the least usage. These almost never sell on market, due to the little demand and huge amount of people who make them for leveling. (I call these the "tree shrooms of alch", done more for experience than actual usage.)
Unlike tree shrooms though, you can sell them to magic shop NPCs and and possibly still profit (if only marginally) even if you buy all the ings. chances of buying the ings are slim to none though, as none of the ings tend to be found on market. Expect to end up harvesting all the ings yourself.
Good for leveling alchemy only, and even then you may be better off even with an essence that gives less experience if you have better access to ings.
I personally also used these in the alch school for power leveling on school days. These days they are the best option for school usage. Yes, you lose the possibility of getting an EME that way, but power leveling is always a tradeoff.
Iron Bar - Commonly made, and will sell, though steel bars will sell much better. Primary audience is people making S2Es for hydrogenium ore runs, and they are more likely looking for steel bars to buy. Despite that, you should be able to get some sales of these.
Matter Conglomerate - A ton of essies, for something that has little demand. Selling them may prove futile since those who actually need them tend to make them themselves. Best made only for yourself, and only when you absolutely need them. Market is almost non-existent.
Steel Bar - Very commonly made, sell quite well thanks mostly to S2E makers. Best made in the thousands, which is quite a lot of time spent on harvesting and making FEs.
Mercury - These days is not worth making at all. Ing cost is way too high, and on top of that the regular breaking of expensive alembics.
Harvy in DP sells them for 10gc each. This really is the best option when you need it in any amounts.
There is no cinnabar on market these days, so if you plan to make it you can expect to harvest that yourself. Very slow, and long trips to storage.
This was, once upon a time, something people used in school on school days for power leveling. However, costs have risen since then, and market has changed. (Add to that, there's regular school days now thanks to day stones, and you're better off keeping a stock of magic essence ings for when school days appear instead.)
Silver Bars - Not as common as iron/steel bars. There is a niche market for these, that being Crafters making medallions and rings. How well they sell tends to fluctuate, so if making these to sell, don't expect a quick sale. Though it may happen.
Generally though, you're better off making them for your own use only as needed. Those who need them also tend to be high enough alchers to mix their own bars.
Gold Bar - Not a big seller. Much lower chance of selling than silver bars, in fact.
Another best only made when you have a personal need for them, that being mainly if you're planning to do a lot of crafting.
Titanium Bar - Extremely low need for these as the items they are needed for are not made in large quantities. Best only made on a self-need basis. Worst case they can be sold to an NPC for 40gc, hardly worth the time involved harvesting the titanium though.
If you have a ton of gc to waste, you can buy these and poof them in alchemy school on school days. (I never did this as my gc was reserved for other skills, but you can technically do this. Much faster than magic essences which I preferred, but a more extreme cost.)
Tin Bar - You will not sell these. At all. Best save them to make bronze bars, which is their only use. Even then, only make on an as-needed basis.
Copper Bar - (I hate to repeat myself, but...) You will not sell these. At all. Best save them to make bronze bars, which is their only use. Even then, only make on an as-needed basis.
Bronze Bar - A combo of tin and copper bars, this is a nightmare to make. Only do so on an as-needed basis. There is almost no market for them, as most who need them can make them themselves.
NPC sales of these (and tin, and copper) also makes them not really worth mixing.
Hydrogenium Bar - Quite regular need for them, particularly due to the ability to buy a Nexus with 50 of them. Most who are buying nexus though do so by harving and making these bars themselves to save gc. Sale-wise you're much better off selling iron/steel bars. High recommendation to make these yourself for nexus-buying though, to save money. Keep in mind the hydrogenium ore itself is quite an annoyance to harvest, and not something everyone can easily do.
(For those who fear doing the Hydro ore runs, there are people who regularly trade your S2Es for their hydro ore, normally at a rate of 11 S2Es to 10 hydro ore, the extra S2E "paying" for their taking the hydro run risk. Consider this as an alternative if you wish to make these bars.)
Seridium Bar - A rise in usage since the addition of the gatherer medallion, and much more as well when harvester medallion was made more useful.
However, as the NPC sells these for 7kgc, and ing cost to self-harv and mix has drastically risen, the majority of these are directly bought from the NPC now.
So... not a good seller.
Wolfram Bar - Don't make. The cost involved is more than the 15kgc the bar costs from the NPC. Save the rare stone used for something else.
Enriched Essences (made with enrichment stone) - Due to the cost of the enrichment stone, the only enriched essence that might be worth making is the EME. Otherwise if you get an enrichment stone, you're better off selling it and buying other enriched essies on market.
Dvarium Bar - Extremely limited need, and the very few people who can actually put these to use likely make them themselves. Don't bother at all unless you're one of those people. Or you are providing ings for a manuer to make something with this bar.
That about covers it. Your mileage may vary, but the above is based on my own experience. Hopefully this provided some insight on what sells and doesn't, or what is just good to make in general even for non-sales.
Comments and opinions welcome. We all experience different things in the course of the game so others may have different experiences with items than I've had.
