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Try sending @Marcia Selsor a pm asking her about this.

Saw dust gives clay a rough surface after it's fired, you can press chips of wood into it when you are finished throwing it for bigger gouges.  I have a pot made by a man and wife team,  they gave everyone a unsigned unglazed small bowl from the seminar.  The top of the bowl has a beautiful tear.  they were using  a Cone 10 stoneware. I think you can mix sawdust in any clay you have.   It has been 20 years ago so my memory may be a little off.    Denice

Hi!
You can definitely create this on your own. I do it in small batches because I don't have a mixer.
Find a ratio of sand, grog, and grit(if you want it super chunky). I sprinkle the mixture onto a wedging surface. Then I take about 5-10 pounds of clay sliced up into cubes. I place the cubes on the mixture and roll them around. Then I slam wedge all the pieces together for a bit, then spiral wedge it and slice it apart into throwing balls.
I will say learning to throw this type of clay takes practice because it will eat your hands apart very quickly. Particularly if you add the grit.
https://www.starworksnc.org/starworks-clay
If you are local to the south east USA. Starworks clay makes a clay that resembles of the above clays:
Grogeewemee 10
This is the coarsest clay we make. It is based on OkeeMedium 10 with added coarse grog.  Perfect for larger scale coil building and slab work. The Grogeewemee 10 clay body works very well in wood or salt kilns.
Cone 10-12, Average Shrinkage at cone 10 OX = 11.94%, Average Water Absorption at cone 10 OX = 3.95%, Contains 19.5% Mullite, Kyanite and coarse grog
I have handled it in person and it is very nice.

@Up in Smoke Pottery and @Marcia Selsor have both been contributors to the forum in the past, although it’s been a while since I’ve seen either of them chime in. Tagging in case they’re lurking.
Marcia is probably one of the foremost obvara artists in North America, and if you get no love from any of the links here, I’d suggest reaching out to her via her website for some clarification points. 

After some searching I found this paper. Which contains some though-provoking facts and ideas.
Recovery and Revival of Attic Vase Decoration Techniques
What can they offer archaeological research ?
https://tinyurl.com/yecn6xnf

Of course the firing (and final effect) of this "black gloss" is different from that of t-sig, but they both start by forming a "colloidal slip" (with or without the use of a deflocculant).

p114 Gives the results of  X-ray analysis of historic samples and modern "reproductions"
- This clearly shows that the originals didn't use phosphorous-based deflocculants  (many reproductions used calgol).
- The "glaze" contains much lower calcium levels than the body.
118 It is pointed out that
- The absence of phosphorous in the glaze implies that  plant-ash deflocculants weren't used.
- Although not conclusive the potassium levels suggest that potash defloculants weren't used either.
 p119  Looking for a deflocculant-free method of producing the "colloidal slip".
- Using a low-calcium clay (similar to that used in antiquity) they found that they could get acceptable (deflocculated?) results a pH>8.7 and >15°C.
- Using the same clay pre-soaked for six months they got similar (not-deflocculated?) results at pH~7 and >15°C.
... this suggests that slip production was a seasonal process, probably during the warmer spring to autumn.
... assisted by over-winter soaking of the clay.
The difference in calcium content between different clays is shown in fig2




PS I couldn't help speculating:
1. That the need for low-calcium clay was because that any soluble calcium compounds would act as deflocculants. After all soluble calcium and magnesium compounds  are used a deflocculants for glaze slips.
2. Historically they may have found suitable candidate clays simply by paddling round the local clay-pits in late spring, looking for signs of spontaneous colloidal dispersion.  Those wishing to find suitable modern commercial clays may need to look at them after a 6-months soak.

Isolating fine particle sizes by allowing clay to settle in water (elutriation, I had to look that up) was known to potters historically, actively used in places where clay had to be “washed” to make it useful. Terra sigillata is an extreme extension of this principle. Natron, naturally occurring soda ash, was well known to the Romans and had many uses, including in ceramics and glass. To me, it’s a stretch to imagine Romans didn’t use a deflocculant of some kind. Looking at the artifacts, I think it’s also important to consider the impact of firing on the finished appearance. Roman terra sig appears different than Greek ware, but it may not all be due to how they made their slip.

Agree 100%! Credit card transaction fees are charged to the seller and should be paid for by the seller. If you're operating a business you should have enough business education to know about COGS (Cost Of Goods Sold). CC fees should be part of those calculations and NOT a method of punishing a certain section of your customer base for using a service YOU are providing. If you don't want to pay CC fees then don't take credit cards. Nuff said.

Agree 100%! Credit card transaction fees are charged to the seller and should be paid for by the seller. If you're operating a business you should have enough business education to know about COGS (Cost Of Goods Sold). CC fees should be part of those calculations and NOT a method of punishing a certain section of your customer base for using a service YOU are providing. If you don't want to pay CC fees then don't take credit cards. Nuff said.

As a consumer, I am insulted when I am charged an additional fee for using my credit card. Credit cards are not a novelty, so sellers should expect that the majority of their customers will be paying with cc.  I feel that the establishment should build this into their expenses. I know what to expect in terms of sales taxes, but I don't want to arrive at the cash register with my credit card out and then be told that I need to pay more, which to me feels like a penalty for using my credit card.
My two cents, which I'll be paying by credit card please.  
Betty
P.S. it use to be that credit card companies prohibited the seller from penalizing the customer and could take away charing privileges, is this no longer true? 

I haven’t, even though I do have some. I just got started with Raku so wanted to do all my learning curve stuff with some reclaim I have which is a scrap mix of B-mix 5 w/wo grog, some Buncombe and other ‘whites’. There’s not a lot of b w/grog in it. So far what I’ve used without adding grog has done well with few cracks; only one explosion so far.