Denim and leather

Update 12/10/13:  This post contains information that is probably incorrect.  Please see here.

We're getting to the fun stuff, how to make your costume and gear - your "kit" as it's called.  We'll start with materials.

The workhorse of ancient clothing fabrics was wool.  Plentiful wherever people raised sheep and relatively easy to process, wool is handsome and hangs smoothly even in light weights, and breathes well.

Unfortunately, if you, like me, happen to be sensitive to it, you don't want it next to your skin all day.  By the time I got to bed on the second day of Marathon 2011, I had welts around the base of my neck that didn't go away for a week.

Lesson:  If you have only one tunic, make it linen.  While it does wrinkle terribly in wear and "run" when washed, it's extremely comfortable, cool, and breathes better than anything.  Linen is made from fibres  from inside the stems of the flax plant, and the long and complicated steps in getting these fibres to the same level of usability as newly-sheared wool makes it very expensive.  It's worth it.  You can wear your linen tunic alone on hot days, or under your wool one or even with just a wool cloak when it's chilly.

Much more expensive in ancient times was cotton, imported from India.  For our purposes, cotton should be regarded as up there with silk, although if you're doing an Indian impression, you have more leeway.  Silk, of course, was also available (via the ancient Silk Road).

I'm still investigating historic weaves.  In the meantime, plain weave is a good default choice.  There will be no thread-counting in XMFM, mainly because I don't know enough about the subject, but also because I do know how hard it is to find good, affordable materials.

The other main kind of cloth used in our period was felt.  Felt is not a weave, but made by compressing loose, tangled wool fibres into a mat.  This mat is water-resistant, can be cut at any angle and won't unravel if you leave the edges unfinished.  It's the material of choice for hats.

Dyes
Get natural-dyed fabric if you can, or dye fabric at home if you're really ambitious.  Madder (red), woad (blue) and turmeric (yellow) were common.  The green rind of Persian (aka English) walnuts produces a greenish-brown dye, but according to Robert Jacobus Forbes (Studies in Ancient Technology), ancient peoples more often used naturally brown and black wool in the production of brown and black fabrics.

While natural dyes tend to fade faster than synthetic ones, this will only make you look like someone who actually wears your costume more than a few times a year.  Replace fabric items if they become so faded as to be unsightly, or as your impression would indicate - the richer your persona, the more clothes you should have and the more frequently you should replace worn ones.  Remember, ancient people liked bold colours and wealthier Persian warriors were known for flaunting their finery on the battlefield.  We are not looking for a drab Hollywood "ancient" look.

If you really want to go all-out as a rich noble or royal bodyguard, wear purple and saffron (golden-yellow or orange).  In Achaemenid times, trade of Tyrian purple was controlled by the state and possession of a garment made with it was an indication of royal favour.  It, and saffron, are as expensive today as they were then because the real things can still only be produced in minute quantities by labour-intensive processes.  While other natural purples are available, they can't replicate Tyrian purple's resistance to fading, so in this case alone would I actually recommend an artificial substitute.

Leather
Leathers should ideally be vegetable- or otherwise naturally-tanned or else just rawhide (not actually leather, but you'll get it from the same sources).  However, I'm not going to insist on anything because finding the right stuff can be challenging.  And you will be needing a lot of leather:

1-2 ounces per square foot:  Almost like fabric, a good leather for covering scabbards or other hard objects.

3-5 ounces:  Use a soft leather in this weight for shoes and as backing for scale armour.

7-9 ounces and up:  All-leather knife sheaths, bowcases, quivers, belts, bottles.

12 ounces and up:  Shoe soles and shields.

Rawhide:  Thick for shields, thin for shield facings.  It's more resistant to puncture than tanned leather, but rots if allowed to stay damp for extended periods.