# Clay & Modeling Materials

Clay starts with how it hardens: air-dry, oven-baked polymer, kiln-fired ceramic, or oil-based clay that never hardens at all. That single fact decides what equipment you need and what the piece can survive afterward. Only fired, lead-free-glazed ceramic is genuinely food-safe; almost everything else, however 'non-toxic' it's labelled, is not.

## How it works

Clay covers four chemistries sharing one name. Ceramic clay is wet aluminosilicate: water lets flat clay particles slide past each other while soft, and firing in a kiln permanently fuses them into stone, a change called vitrification that can't be undone. Polymer clay is PVC resin, plasticizer and pigment; it never dries on its own, because heat, not evaporation, fuses the particles solid in a home oven. Air-dry clay is a fine mineral or cellulose paste that hardens as its water evaporates at room temperature, a physical change reversible with enough water right up until it sets. Oil-based clay (plastilina) suspends clay or wax in oil that never evaporates, so it stays workable indefinitely and gets reused rather than finished. Two-part epoxy clay hardens by a chemical reaction once its two components are mixed. [3] [6] [7] [8] [10] [14]

| Family | What it is | Suits |
|---|---|---|
| Ceramic / pottery clay (kiln-fired) | A natural clay body fired in a kiln until it vitrifies into stone; the only family that can become genuinely food-safe, with the right lead-free glaze. | Mugs, bowls, vessels, hand-building and wheel work, anyone who owns or has access to a kiln. |
| Polymer clay (oven-bake) | PVC resin that stays soft indefinitely until baked solid in a home oven; huge color range, holds fine detail. | Jewelry, miniatures, figurines, mixed media; no kiln needed. |
| Air-dry clay | Hardens on its own at room temperature over hours to days; no oven or kiln, lighter and more break-resistant once dry than it feels while wet. | Kids' projects, classrooms, keepsakes, lightweight decor. |
| Oil-based clay (plastilina / plasteline) | Never hardens at all; stays workable forever and gets reused rather than kept as a finished piece. | Sculptors' studies, stop-motion animation, product-design maquettes, mold-making masters. |
| Two-part epoxy clay | Two components mixed by hand that cure hard through a chemical reaction, with no oven or kiln. | Repairs, mixed media, jewelry findings, prop and model making. |

## How an expert narrows the choice

```mermaid
timeline
  section 1 · Cure method
    oven-bake : ask Firmness when fresh, Bake temperature, Bake time per 1/4 inch : skip Clay body, Firing range, Sulfur content
    air-dry : skip Clay body, Firing range, Sulfur content, Bake temperature, Bake time per 1/4 inch
    kiln-fired : ask Clay body, Firing range, Food-safe once finished : skip Sulfur content, Bake temperature, Bake time per 1/4 inch, Firmness when fresh
    stays soft forever : ask Firmness when fresh, Sulfur content : skip Clay body, Firing range, Bake temperature, Bake time per 1/4 inch, Food-safe once finished
    two-part epoxy : skip Clay body, Firing range, Sulfur content, Bake temperature, Bake time per 1/4 inch
  section 2 · Food-safe once finished
    Will it ever hold food or drinks?
  section 3 · ACMI safety seal
    Mainly for a child in sixth grade or younger?
```

1. **Cure method.** Air-dry, oven-bake, kiln-fired, stays soft forever, or two-part epoxy?
   Hardening method decides whether you need an oven, a kiln, nothing at all, or a mixed epoxy, and it rules out most of the catalog before any other question.
   If oven-bake: ask Firmness when fresh, Bake temperature, Bake time per 1/4 inch; skip Clay body, Firing range, Sulfur content.
   If air-dry: skip Clay body, Firing range, Sulfur content, Bake temperature, Bake time per 1/4 inch.
   If kiln-fired: ask Clay body, Firing range, Food-safe once finished; skip Sulfur content, Bake temperature, Bake time per 1/4 inch, Firmness when fresh.
   If stays soft forever: ask Firmness when fresh, Sulfur content; skip Clay body, Firing range, Bake temperature, Bake time per 1/4 inch, Food-safe once finished.
   If two-part epoxy: skip Clay body, Firing range, Sulfur content, Bake temperature, Bake time per 1/4 inch.
   Skip if already known: an earlier answer naming polymer clay, air-dry clay, pottery/ceramic clay, or plastilina/modeling clay by name.

2. **Food-safe once finished.** Will it ever hold food or drinks?
   Only fired, lead-free-glazed ceramic qualifies. A yes answer rules out polymer, air-dry and oil-based clay outright, however non-toxic their labels say they are.
   Skip if already known: cure_method already answered as anything other than kiln-fired.

3. **ACMI safety seal.** Mainly for a child in sixth grade or younger?
   ACMI's AP seal is certified safe at that age; its CL seal must never go to a child that young. A yes answer should filter to AP-sealed products only.
   Skip if already known: memory already states the buyer is shopping for a young child or for themselves as an adult.

## What to surface

### Cure method

How the clay goes from workable to finished: air-drying at room temperature, baking in a home oven, firing in a kiln, mixing a two-part epoxy, or never hardening at all. It decides the equipment you need (none, an oven, or a kiln), how permanent the result is, and what else about the clay even applies. Polymer clay's color range and caning techniques mean nothing to a stoneware buyer; a stoneware buyer's cone and glaze questions mean nothing to someone with a bag of plastilina that will never see heat at all. [3] [6] [7] [8] [10]

Values: air-dry, oven-bake, kiln-fired, stays soft forever, two-part epoxy.
Question to ask: Air-dry, oven-bake, kiln-fired, stays soft forever, or two-part epoxy?

Used as: top-level split, filter. Data: derived from listing fields, backfill cost low. Look for it as: Not a labeled field on most listings; infer from the product title and category (polymer clay, air-dry clay, pottery/stoneware clay, plastilina/modeling clay).
Almost always clear from the title or category name. Ambiguous only for generically titled 'modeling clay' listings, which need the description read to tell oven-bake from oil-based.
Chart: Hardening-method map.

### ACMI safety seal

ACMI's own certification mark: AP means certified non-injurious for any age including young children; CL means safe only for someone old enough to read and follow a caution label. This is the actual governing standard for child safety in art materials, not a general 'non-toxic' claim. ACMI states the CL seal 'should never be given to children in grade 6 or lower,' so a product's seal, not its marketing copy, is what a teacher or parent should filter on. [1] [2]

Values: ap_seal, cl_seal, none_stated.
Question to ask: Mainly for a child in sixth grade or younger?

Used as: top-level split, filter, shown on the card. Data: needs research, backfill cost medium. Look for it as: Rarely a structured field. Printed on the package as a circular AP or CL mark; checkable against ACMI's own certified-product search tool by brand and product name.
Check the package photo for the AP/CL mark first; if absent, search ACMI's certified-product tool by brand and product name before marking none_stated.

### Food-safe once finished

Whether the cured or fired piece is safe for food or drink to touch, which needs a fully vitrified, lead-free-glazed ceramic body, not just a 'non-toxic' label. Buyers assume a clay labelled non-toxic is also food-safe; it usually is not. Non-toxic means safe to handle; food-safe means a glaze and firing combination that won't leach lead or cadmium into what's eaten off it, which only applies to kiln-fired ceramic, and only when the maker states it. No single food-contact regulator certifies this directly; the claim rests on art-material labeling law (ACMI, CPSIA) and glaze-firing guidance, not a dedicated FDA/CPSC ceramicware food-safety standard. [2] [11]

Question to ask: Will it ever hold food or drinks?

Used as: top-level split, filter. Data: needs research, backfill cost medium. Look for it as: Rare on retail listings. Look for an explicit 'food safe' or 'lead-free glaze' claim on the brand's own clay-body or glaze page, not the clay bag alone.
False by rule for every cure_method except kiln-fired, including two-part epoxy. For kiln-fired clay, confirm the maker states a lead-free, food-safe glaze; do not infer it from a general non-toxic claim.

### Firmness when fresh

How much hand strength the clay takes to knead soft before shaping it, from straight-out-of-the-pack soft to firm enough to resist blurring fine detail. Soft clay suits kids and beginners and is forgiving of weak hands; firm clay holds crisp edges and supports advanced techniques like caning and mokume gane but takes real effort to condition, and softens further the more you work it. [5] [6] [9]

Values: soft, medium, firm.
Question to ask: Soft and easy to knead, or firm for fine detail?

Used as: shown on the card, filter. Data: derived from listing fields, backfill cost low. Look for it as: Not a labeled field on most listings; infer from the product line name (Sculpey III, FIMO soft = soft and beginner-aimed; Premo, FIMO professional = firm and detail-aimed) or the description.
Map known line names to firmness directly; only unfamiliar or store-brand lines need the description read.

### Clay body

The mineral makeup of a kiln-fired clay: earthenware (low-fire, stays porous unless glazed), stoneware (mid-to-high-fire, vitrifies dense) or porcelain (high-fire, white and translucent-thin). It sets the firing range the piece needs, how watertight it gets without glaze, and what glazes are even compatible, since a glaze formulated for one body can crawl or craze on another. [10]

Values: earthenware, stoneware, porcelain.

Used as: shown on the card, filter. Data: on the listing, backfill cost low. Look for it as: Stated directly in the clay's product name, e.g. 'red earthenware', 'stoneware clay', 'porcelain clay body'.
Name-match against the three terms; near-universal on ceramic clay listings since it is the clay's actual product category.

### Firing range

The band of kiln heat, measured in Orton pyrometric cones, that a ceramic clay and its glaze are rated to: low-fire, mid-fire, or high-fire. A clay body and a glaze only work together if they are rated to the same cone range; firing a low-fire clay to a high-fire cone can warp or melt it, and under-firing leaves a porous, food-unsafe body even under a glaze. [13] [11] [12]

Values: low_fire, mid_fire, high_fire.

Used as: shown on the card, filter. Data: derived from listing fields, backfill cost medium. Look for it as: Given as a cone number or range on the clay bag or glaze bottle (e.g. 'Cone 06' or 'Cone 5–6'), rarely transcribed onto the online listing.
Derive from the clay_body_type and the stated cone number where present; AMACO's own glaze pages print cone-to-degree equivalents for reference.
Chart: Orton cone scale.

### Sulfur content

Whether an oil-based modeling clay is made with a sulfur-containing compound or is sulfur-free. Sulfur-based oil clay can stop some silicone mold rubbers from curing where it touches them, ruining a mold. Sculptors making a piece that will be cast reach for a sulfur-free clay specifically so it can go straight into a mold. [9]

Values: sulfur_free, sulfur_based.

Used as: shown on the card, filter. Data: needs research, backfill cost medium. Look for it as: Not usually a structured field. Check the brand's own product description for 'sulfur-free', common wording on lines marketed for mold-making.
Niche but important when it matters. Default unknown oil-based clays to sulfur_based unless the maker states sulfur-free, since the classic studio formulas are sulfur-based.

### Bake temperature

The oven temperature an oven-bake (polymer) clay needs to cure solid, as printed on its package. There is no single number: it varies by brand and line, and baking at the wrong temperature either leaves the piece crumbly and uncured or scorches and discolors it. [4] [6]

Range: 230 to 275 °F. follow the specific package; most brands fall between 230°F and 275°F (110–135°C).

Used as: shown on the card. Data: needs research, backfill cost low. Look for it as: Printed on the physical package, not usually transcribed onto the online listing; check the brand's own instructions page instead.
Printed on every package; a quick look-up per brand and line, not per SKU.

### Bake time per 1/4 inch

Minutes in the oven per 1/4 inch (6 mm) of thickness, the unit Sculpey's own instructions use for oven-bake clay. A flat bake time undercures a thick piece; scaling the time to thickness is how an expert avoids a soft, crumbly center in anything bulkier than a thin bead or sheet. [4]


Used as: shown on the card. Data: needs research, backfill cost low. Look for it as: Printed on the package alongside the bake temperature, not on the online listing.
Sourced to Sculpey's own documented method (30 min per 1/4 in); other brands' packaging may state a flat time instead of a per-thickness rule, so check before assuming the same scaling applies.

## Numbers worth computing

| Metric | Formula | Why |
|---|---|---|
| Total bake time for this piece (minutes) | `(thickness_in / 0.25) * cure_time_min_per_quarter_in` | Sculpey's own instructions scale bake time to thickness, 30 min per 1/4 in; a thicker piece needs proportionally longer, not a flat default time, or the center stays uncured. Other brands may print a flat time instead, so confirm the specific package before relying on this formula. [4] |

## What you give up

- Firm, detail-holding clay such as Premo keeps sharp lines and crisp caning but takes real hand strength to knead soft; clay aimed at kids and beginners trades that detail for a clay that's soft straight out of the pack. [5] [6] Board branch: "See more beginner-firmness clay".
- Sulfur-based plastilina, the classic studio clay sculptors have used since the 1950s, can stop some silicone mold rubbers from curing against it. Sulfur-free formulas exist specifically so a sculpted piece can go straight into a mold. [9] Board branch: "See more sulfur-free sculpting clay".
- Only a fired, lead-free-glazed ceramic piece is genuinely food-safe and dishwasher-tough, but it needs a kiln most buyers don't own, and the clay shrinks as it dries and fires, so the finished piece ends up smaller than what was made. [11] [2] Board branch: "See more food-safe ceramic clay".
- Polymer clay cures the same day in a home oven and never dries out while you work it, but the cured piece stays heat-sensitive and, unlike fired ceramic, is not considered food-safe for anything that regularly touches food or drink. [3] [2]

## How to draw it

**Orton cone scale** (scale, shows Firing range). A single line from the coolest cone to the hottest that every ceramic clay, glaze and kiln is rated against, so a buyer can see at a glance whether a clay and a glaze will actually fire together. Used today by The Edward Orton Jr. Ceramic Foundation; printed on nearly every ceramic clay and glaze bag.. [13] [11]

**Hardening-method map** (two axis map, shows Cure method). Plots each cure method by the equipment it needs (none, oven, kiln) against how permanent the result is, so a buyer with no oven or kiln sees at a glance which part of the map is open to them. This is a proposal, nobody draws it this way yet. [3] [7] [8]

## Words you will meet

- **Cone**. A small ceramic pyramid that bends at a known combination of kiln temperature and time, proving a firing actually reached the heat a clay or glaze needed.
- **Vitrify**. The permanent, glass-like fusing that happens inside a kiln above a clay's firing point; a fired, vitrified piece can never go back to being clay.
- **Condition**. Knead clay by hand or through a pasta machine until it's soft and bend-flexible before shaping it.
- **Armature**. An internal support, often crumpled foil, built inside a thick piece so it hollows out, cures evenly and doesn't crack.
- **AP seal / CL seal**. ACMI's two safety labels: AP means certified safe for any age including young children; CL means safe only for someone old enough to read and follow a caution label.
- **Plastilina / plasteline**. The trade name for oil-based, never-hardening modeling clay, reused indefinitely rather than finished into a permanent piece.
- **Sulfur-based**. An oil clay made with a sulfur-containing compound, which can stop some silicone mold rubbers from curing where it touches.
- **Bisque**. Ceramic ware fired once, hard but not yet glazed or waterproof, ready to be glazed and fired a second time.

## Where we made a call

- Oven-bake cure temperature and time vary by brand; there is no single universal number. (confidence high). FIMO soft hardens at 110°C (230°F); Sculpey Premo at 275°F (130°C). Always follow the specific package, not a remembered number. [6] [4]
- Most clay, however it's labelled, is not food-safe for things that regularly touch food or drink. (confidence medium). Based on ACMI's LHAMA/CPSIA material and AMACO's lead-free glaze guidance; FDA and CPSC's own ceramicware pages would not load for direct verification. [2] [11]
- Sulfur-based oil clay can stop some silicone mold rubber from curing against it. (confidence medium). Chavant calls the two 'incompatible' and sells sulfur-free lines as mold-safe alternatives. Single-sourced; no independent source confirms the specific cure-inhibition mechanism. [9]
- ACMI's CL caution seal must never go to a child in sixth grade or younger. (confidence high). Only the AP seal is certified safe at that age; ACMI states CL appears on no children's materials at all. [1]
- A microwave cure path is not a mainstream option for oven-bake clay. (confidence medium). Sculpey's own instructions say 'NEVER put oven-bake clay in the microwave.' The add-on schema's microwave value needs a named product before it's kept. [3]
- Two-part epoxy clay cures permanently hard; stretchy toy putty like Silly Putty never does. (confidence high). This is the line between this register and the play_dough sibling: epoxy putty is a thermosetting polymer that goes rigid, toy putty stays pliable by design. [15]
- Two-part epoxy clay's mix ratio and no-oven, no-kiln cure are confirmed; its sulfur content and firmness are not. (confidence low). avesstudio.com (Apoxie Sculpt) would not load; Wikipedia's general epoxy-putty article backs the 1:1 mix and room-temperature chemical cure, nothing brand-specific or about sulfur or firmness. [14]

## References

1. [ACMI — What Do the Seals Mean? (Art Materials Safety)](https://acmiart.org/materials-safety) (standards body). Supports: AP seal is certified safe for children sixth grade or younger; CL seal must never go to a child that young; Both seals certify to ASTM D4236 and LHAMA.
2. [ACMI — Governing Regulations (LHAMA, CPSIA, ASTM D4236)](https://acmiart.org/regulations) (regulator). Supports: LHAMA requires chronic-hazard labeling under ASTM D4236; CPSIA lowered lead limits in children's products; CPSC enforces art-material labeling law.
3. [Sculpey — What to Know About Polymer Clay](https://www.sculpey.com/pages/new-to-polymer-clay) (spec sheet). Supports: Polymer clay stays soft until baked in a home oven; Never microwave oven-bake clay; Pieces thicker than 1/4 inch need an armature.
4. [Sculpey Premo Bead Making Kit — Basic Instructions](https://cdn.shopify.com/s/files/1/0667/6421/0426/files/PE_4048_Premo_Bead_Making_Kit.pdf) (spec sheet). Supports: Bake Premo at 275°F (130°C) for 30 minutes per 1/4 inch thickness; Do not microwave or overbake.
5. [Sculpey Clay Comparison Chart](https://cdn.shopify.com/s/files/1/0667/6421/0426/files/ClayComparisonChart_1.2026_OL.pdf) (spec sheet). Supports: Sculpey III is soft, matte, aimed at kids and beginners; Premo is firm, satin-finish, for caning and detail work; A wax-based non-hardening line exists for claymation and play.
6. [STAEDTLER — FIMO soft](https://www.staedtler.com/intl/en/products/fimo-modelling-clay-accessories/fimo-soft/) (spec sheet). Supports: FIMO soft hardens in an oven preheated to 110°C for 30 minutes; Fully hardened only once cooled.
7. [STAEDTLER — FIMOair](https://www.staedtler.com/intl/en/products/fimo-modelling-clay-accessories/fimoair/) (spec sheet). Supports: FIMOair hardens by air drying at room temperature, no oven or kiln; Drying takes hours to days depending on thickness; Forcing drying with heat or sun can crack it.
8. [Chavant Inc. — Professional Oil Based Clays](https://chavant.com/) (spec sheet). Supports: Oil-based clay (plastilina) is used by fine art and industrial sculptors; Chavant has made it since 1892.
9. [Chavant — Fine Art Sculpting Clays](https://chavant.com/fine-art-sculpting-clays) (spec sheet). Supports: Sculptex is sulfur-free, an alternative to sulfur-based clays incompatible with mold rubbers; Castilene comes in medium and hard detail grades.
10. [AMACO — clay shop category navigation](https://www.amaco.com/) (spec sheet). Supports: AMACO separates ceramic, air-dry, self-hardening and oil-based modeling clay as distinct lines.
11. [AMACO — Low Fire Gloss (LG) Glazes](https://shop.amaco.com/glazes-underglazes/low-fire-glazes/lg-low-fire-gloss/) (spec sheet). Supports: Cone 05 glaze fire is 1888°F; Cone 04 bisque fire is 1945°F; Always slow-fire lead-free glazes.
12. [AMACO — Potter's Choice (PC) Glazes](https://shop.amaco.com/glazes-underglazes/high-fire-glazes/pc-potters-choice/) (spec sheet). Supports: Cone 5 is 2167°F and Cone 6 is 2232°F; Bisque fire is Cone 04, 1945°F.
13. [The Edward Orton Jr. Ceramic Foundation — Pyrometric Cones](https://www.ortonceramic.com/pyrometric-cones) (standards body). Supports: Pyrometric cones measure heat-work, the combined effect of time and temperature; Used worldwide to monitor kiln firings.
14. [Wikipedia — Epoxy Putty](https://en.wikipedia.org/wiki/Epoxy_putty) (primer). Supports: Two-part epoxy putty mixed 1:1 cures hard through an exothermic chemical reaction; Hardened epoxy putty keeps its shape when heated, unlike oven-bake clay.
15. [Wikipedia — Putty](https://en.wikipedia.org/wiki/Putty) (primer). Supports: Play putty such as Plasticine and Silly Putty stays pliable as a toy; Epoxy putty permanently hardens after curing, unlike play putty.