# Firing

Sculpture I · Modelling, Moulds and the Workshop · https://tryals.app/learn/sculpture-i/firing

## Two Firings, Two Jobs

Most studio ceramics passes through the kiln twice. The two firings do different things.

**Bisque firing** converts raw clay to ceramic. It burns out organics and drives off chemical water. This leaves a **porous** body strong enough to handle and glaze. Its typical range is **950 to 1000 °C** (950, 1000).

**Glaze firing** melts the glaze into glass fused to the body. It also vitrifies the clay. The temperature depends entirely on the clay body:

| Body | Typical glaze firing | Result |
|---|---|---|
| **Earthenware** | 1000–1100 °C | Porous body; glaze provides the seal |
| **Stoneware** | 1200–1300 °C | Vitrified, strong, waterproof |
| **Porcelain** | 1280–1400 °C | Vitrified, translucent when thin |

The **first** firing is usually the cooler one. Bisque only needs to make ware porous. Glaze firings must melt glass. Bisquing too high prevents the body from absorbing glaze properly.

### Schedule, Cones, and Atmosphere

A kiln firing is a **schedule**, not a target. Three points need care:

- **Below 200 °C**: physical water becomes steam. Firing too fast breaks damp ware.
- **Around 573 °C**: **quartz inversion**. Silica suddenly changes volume. Rushing through 573 °C cracks work on the way up and on cooling.
- **Near the top**: soaking evens out heat and matures the glaze. Ceramics responds to **heat work**—temperature over time—not peak temperature alone.

**Pyrometric cones** measure actual heat work. These calibrated pillars bend at specific heat work values, unlike single-point thermocouples.

Kiln atmosphere also alters results:

- **Oxidation**: plentiful oxygen. Standard in electric kilns; colours stay predictable.
- **Reduction**: oxygen-starved. Fuel flames pull oxygen from the clay and glaze, shifting colours.

A glaze recipe only gives a colour **under a stated atmosphere and temperature** (between 300 °C and peak).

> **Common pitfall:** treating a kiln like an oven with a single target temperature. Schedule, atmosphere, and heat work determine the final piece.

## Practice questions

10 of this lesson's 14 practice questions, with answers. The full set is in the app.

### 1. Why do pyrometric cones remain in use alongside electronic temperature control?

A. Cones measure internal ambient temperature with far greater precision than digital thermocouples
B. Electronic kiln controllers cannot reliably operate at temperatures required for stoneware and porcelain
C. A cone measures heat work, temperature and time together, as the ware actually experienced it
D. Cones indicate whether an oxidation or reduction atmosphere has been successfully maintained inside

**Answer:** C. A cone measures heat work, temperature and time together, as the ware actually experienced it

**Why:** Because ceramics responds to **heat work** rather than peak temperature, and a thermocouple reads one temperature at one point in the chamber. A cone is formulated to bend at a given amount of heat work and sits with the ware, so it reports what the pieces actually experienced.

Page: https://tryals.app/practice/sculpture-i/firing/why-do-pyrometric-cones-remain-in-use-alongside-electronic

### 2. Which are true of quartz inversion at about 573 degrees?

A. It marks the point at which glaze begins to melt
B. It occurs on heating and again on cooling
C. Passing it too quickly in either direction can crack the work
D. Silica in the body changes crystal form with a sudden volume change

**Answer:** B. It occurs on heating and again on cooling; C. Passing it too quickly in either direction can crack the work; D. Silica in the body changes crystal form with a sudden volume change

**Why:** The second point is the one that costs people firings. The inversion happens on the way **down** as well as up, so opening a kiln early to see the results is a reliable way to crack a load that survived the climb perfectly.

Page: https://tryals.app/practice/sculpture-i/firing/which-are-true-of-quartz-inversion-at-about-573-degrees

### 3. The same glaze recipe produces the same colour regardless of kiln atmosphere.

**Answer:** False

**Why:** **False**, and dramatically so. Copper gives green in oxidation and red in reduction; iron behaves entirely differently in the two. A glaze recipe is a colour **under a stated atmosphere and temperature**, and the same recipe fired differently is a different glaze.

Page: https://tryals.app/practice/sculpture-i/firing/the-same-glaze-recipe-produces-the-same-colour-regardless-of-kiln

### 4. Sort each statement by whether it concerns the BISQUE firing or the GLAZE firing.

**Answer:**

- Bisque firing: Drives off chemically combined water, Leaves the body porous enough to absorb glaze, Burns out organic material
- Glaze firing: Melts a glass layer fused to the body, Takes a stoneware body towards vitrification, Is usually the hotter of the two

**Why:** The two firings have genuinely different jobs, which is why the order of temperatures surprises people. The first makes the body workable-with; the second has to melt glass, and that takes considerably more heat.

Page: https://tryals.app/practice/sculpture-i/firing/sort-each-statement-by-whether-it-concerns-the-bisque-firing-or-the

### 5. Complete the account of firing.

**Answer:** The first firing, which converts clay to ceramic and leaves it porous, is the **bisque** firing. Silica changes crystal form at about **573** degrees, on the way up and again on the way down. A kiln starved of oxygen is firing in **reduction**. Ceramics responds to heat **work** rather than to peak temperature alone.

**Why:** The last blank is the concept that makes the rest coherent. Two firings reaching the same peak by different schedules give different results, which is why a firing is specified as a schedule and measured with cones rather than named by a number.

Page: https://tryals.app/practice/sculpture-i/firing/complete-the-account-of-firing

### 6. A kiln firing is defined as an entire schedule rather than reaching a single peak temperature. What practical consequence follows from this distinction when managing the final stages of a firing?

A. Reaching the target temperature instantly completes the glaze melt
B. Holding top heat allows glazes to mature via cumulative heat work
C. Peak temperature must be held indefinitely to prevent thermal shock
D. Rushing the final climb ensures the pyrometric cones remain upright

**Answer:** B. Holding top heat allows glazes to mature via cumulative heat work

**Why:** Peak temperature alone does not mature ceramic ware; material transformation depends on heat work accumulated over time. Cones track this combined effect of duration and heat, whereas holding indefinitely over-fires ware or causes defects rather than preventing thermal shock.

Page: https://tryals.app/practice/sculpture-i/firing/a-kiln-firing-is-defined-as-an-entire-schedule-rather-than-reaching-a

### 7. At approximately what temperature, in degrees Celsius, does quartz inversion occur?

**Answer:** 573 (within ±20)

**Why:** **About 573 °C.** It is worth memorising as a number rather than a range, because it is the one point in a firing schedule where the rate matters more than anything else, in both directions.

Page: https://tryals.app/practice/sculpture-i/firing/at-approximately-what-temperature-in-degrees-celsius-does-quartz

### 8. Match each firing atmosphere or concept to what it means.

**Answer:**

- Oxidation → Plenty of oxygen; the default in an electric kiln
- Reduction → Oxygen starved, so the flame draws it from body and glaze
- Soaking → Holding temperature so glaze matures and heat evens out
- Heat work → Temperature and time together, which is what ware responds to

**Why:** Pairing atmosphere with schedule is the point: a firing is specified by both, and neither alone predicts the result. That is why a glaze recipe travelling between studios so often disappoints, the recipe moved and the firing did not.

Page: https://tryals.app/practice/sculpture-i/firing/match-each-firing-atmosphere-or-concept-to-what-it-means

### 9. Arrange the stages of a firing schedule in order.

**Answer:**

1. Slow climb below 200 degrees to drive off residual water
2. Careful passage through quartz inversion at about 573 degrees
3. Climb to the target for the body and glaze
4. Hold at the top of the schedule before cooling begins
5. Controlled cooling, again slowly through 573 degrees

**Why:** The inversion appearing at both ends of the schedule is the shape worth remembering. Everything between them is about reaching enough heat work; everything at them is about not cracking the ware on the way past.

Page: https://tryals.app/practice/sculpture-i/firing/arrange-the-stages-of-a-firing-schedule-in-order

### 10. A body bisque-fired too high may fail to absorb glaze properly.

**Answer:** True

**Why:** **True.** The bisque firing is meant to leave the body **porous** so that glaze applied as a water suspension is drawn in and holds. Fire it too high and the body starts to vitrify, the porosity drops, and the glaze runs off or sits too thin, which is why the first firing is deliberately the cooler one.

Page: https://tryals.app/practice/sculpture-i/firing/a-body-bisque-fired-too-high-may-fail-to-absorb-glaze-properly