Leathercraft Supplies: what Claude and GPT would buy

Two AI models chose on their own, with no web access. Nobody tested these products: this is what each one says, and how to check it.

They disagree: Claude chooses Julius Koch and GPT chooses Fiebing's

In Arts, Crafts & Sewing, Claude and GPT choose the same brand in 46 of 83 categories.

Claude Opus 5.5 · October 27, 2026

Julius Koch Ritza 25 Tiger Thread (line)

A pre-waxed braided polyester thread that holds a tight, even saddle stitch without fraying and is the benchmark most hand-stitchers measure other threads against.

Not for: Machine stitchers, and anyone lacing with leather strips rather than sewing with thread.

GPT 6.1 Sol · November 10, 2026

Fiebing's Pro Dye (line)

Produces rich, penetrating color on vegetable-tanned leather, making it a versatile choice for belts, wallets, and other leathercraft projects.

Not for: Suede, synthetic leather, or projects requiring a water-based dye.

Which one fits you

cheaper waxed thread for practice

Tandy Leather (brand)

Tandy's waxed threads are widely available and adequate while you learn the saddle stitch.

Not for: Makers who want the flattest, most consistent stitch line on finished goods.

Check before you buy

Thread diameter vs. stitching iron spacing. Pair finer thread (around 0.45–0.6 mm) with tighter iron spacing (around 3 mm) and heavier thread (0.8 mm and up) with wider spacing (4 mm and up). Thread that is too thick for the holes crowds and distorts the stitch; thread that is too thin looks sparse and loose.
Leather weight. Leather is sold in ounces, where 1 oz is about 1/64 in (about 0.4 mm). Small goods usually use 2–4 oz and belts or holsters 7–10 oz. The weight sets which rivets, needles and thread will fit the project.
Tanning type. Use vegetable-tanned leather for tooling, stamping, burnishing and dyeing; use chrome-tanned for soft, flexible goods. Chrome-tanned leather will not hold tooling impressions or burnish properly.

Where to find Julius Koch Ritza 25 Tiger Thread

Rocky Mountain Leather Supply
eBay

Shops seen in search results on November 5, 2026. A shop missing here may sell it too. Nothing was checked by a person.

Brands that matter

Tandy Leather: The broadest one-stop source for leather, tools, hardware and lace.
Julius Koch (Ritza): Maker of Tiger thread, the reference thread for hand-stitched leather.
Seiwa: Japanese maker of Tokonole burnishing gum and other finishing products.
Weaver Leather Supply: A source for heavier tools and hardware for saddlery and tack.

What to avoid

Mixed starter kits whose stitching chisels are a different spacing from what the included thread suits.
Leather sold with no thickness or tannage stated.

Questions

Waxed thread or plain thread plus beeswax?

Pre-waxed braided polyester is simpler and more consistent. Waxing linen yourself is traditional, but it takes practice to get right.

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