Firing costs by weight or by kiln space? The debate, settled fairly

In every studio the question comes up at some point, usually over a specific piece. A large, thinly thrown bowl weighs 400 grams and takes up half a shelf in the kiln. Next to it stands a small, solid candle holder of the same weight that fits into any gap. Billed by weight, both pay the same price. Whoever made the bowl finds that fair. Whoever loads the kiln does not.

Billing firing costs by weight or by kiln space is therefore one of the oldest debates among studios, and forums revisit it regularly. This post takes both sides seriously, looks at what studios actually do, and shows how three simple rules settle the question.

What a firing really costs

At the heart of the debate is one observation: a firing costs almost the same whether the kiln is full or half full. Electricity, time and wear depend on temperature and duration, not on the weight of the load. The scarce resource is the space on the shelves, not the mass of the pieces. If you want to know exactly what a single firing costs, the post on the electricity cost of a kiln has the calculation.

If space is the scarce resource, space would be the logical pricing basis. That almost all studios weigh anyway has good reasons.

The weight camp

Weight is the most practical measure there is. A scale next to the kiln, one number, done. Nobody has to argue, every figure can be checked, and the billing at month end is a multiplication.

On top of that, for the large majority of pieces weight is a decent stand-in for space. Mugs, bowls, cups, small vases, the daily bread of most studios, have a similar ratio of weight to footprint. For them the kilo represents space well enough.

And weight sets a good incentive. Whoever throws thinner pays less. Throwing thin is craft a studio wants to encourage.

The weaknesses of weight are well known and always the same:

• Plates and platters. They weigh little but occupy a whole level. A set of plates can half fill a kiln and brings in almost nothing on the scale.
• Large, thin-walled pieces. The bowl from the opening, tall slender vases, lampshades.
• Bulky shapes. Sculptural work, pieces with protrusions, anything that leaves no room around it.
• Height. A tall piece costs a whole shelf level, because the next shelf has to be set higher. The scale sees none of that.

The space camp

Whoever bills by space hits the costs more accurately. The only question is how to measure space, and that is where the model usually fails in practice. The variants that occur:

• By shelf level. The kiln is thought of in levels, each level has a price, a piece occupies a level or a share of it. Clear for large pieces, vague for small ones.
• By footprint. Length times width of the piece in centimetres, sometimes times height as volume. Objective, but every piece has to be measured, and irregular shapes make it hard.
• By size class. Small, medium, large, each with a fixed price. Quick at the counter, but the borderline cases get debated.
• Whole kiln. A fixed price for the entire firing, the piece fills it however it likes. Common for outside customers and large projects, not for members day to day.

The shared problem: space is an estimate, weight is a measurement. As soon as estimating starts, there are conversations at the counter, and the person loading the kiln becomes the referee. For a studio with many members and changing staff that is more expensive over time than any inaccuracy of the scale.

What studios actually do

Looking at published price lists, a clear picture emerges: almost all studios weigh, and almost all correct the scale at exactly the points where it fails. The corrections are always the same:

• A minimum price per piece, so small items do not fall below handling costs.
• A fixed price for plates, in practice around 4 to 10 euros per piece, regardless of weight.
• A size rule for large, light pieces, where above a certain size the price is calculated by volume or by class instead of by weight.
• Kiln rental for anything that fills a firing on its own, in practice around 50 to 190 euros depending on the kiln.

So there are not really two camps but one practice: weight as the basis, space as the exception. The complete guide to firing costs lists these rules among the ten adjustments that sit above the base model.

The three rules that account for space

Whoever wants to end the debate in their own studio does not need a new pricing basis. Three rules on top of the per-kilo price cover almost every case:

1. Minimum price per piece. Catches the pendant, the button and the test tile. The price is the maximum of weight times rate and the minimum amount. Typical values are 2 to 5 euros.

2. Fixed price for categories. Plates, platters, coasters get a per-piece price instead of a per-kilo price. The list stays short: only the shapes where the scale is systematically off.

3. Size class or kiln rental for special cases. Anything that occupies a whole level or a whole kiln is agreed beforehand and paid by class or by firing. It happens rarely, but it has to be settled before the piece is on the shelf.

As a fourth, half rule, rounding up to the next full kilo. It is not a measure of space, but it evens things out a little for light pieces and makes the maths at the counter easier.

A worked example

A studio bills the glaze firing at 8 euros per kilo, has a minimum price of 3 euros and a fixed price of 5 euros for plates.

• A mug at 400 grams: 0.4 times 8 is 3.20 euros. Above the minimum, so 3.20 euros.
• A plate at 400 grams: arithmetically also 3.20 euros, but the category applies, so 5 euros. The plate occupies many times the space, the surcharge is moderate and justifiable.
• A pendant at 40 grams: arithmetically 0.32 euros, the minimum price turns it into 3 euros.
• A 60-centimetre vase at 1.8 kilos: arithmetically 14.40 euros. But it costs two shelf levels. Rule 3 applies here, and the studio agrees a class price in advance, say 25 euros.

Four pieces, four clean prices, and no discussion while loading. With your own rates you can play this through in the firing-cost calculator.

How to decide for your studio

• Class studio, mostly beginner pieces: weight plus minimum price is enough. Plates are rare, special cases rarer still.
• Open studio with experienced members: weight plus all three rules. This is where the big thin bowls and the plate sets come from.
• Firing service for outside customers: weight plus minimum price for walk-ins, kiln rental for anything large. The price is stated at intake, which defuses the space question by itself.

Whichever variant: explain the why to members. “The kiln costs per firing, not per kilo, which is why plates have a fixed price” is a sentence everyone understands. Without it, every exception looks arbitrary.

The model has to be billable

The three rules are quickly written down. Harder is applying them consistently every month, for every piece, in every firing, in a spreadsheet that knows all the exceptions. That is exactly where many studios give up the space argument, not because it is wrong, but because the billing becomes too tedious.

In WheelOwl weight is the basis, and the rules sit on top as switches: minimum price, rounding up to the next full kilo, and fixed surcharges or fixed prices per piece for plates and other categories. You set them up once, and firing-cost billing applies them to every piece without anyone recalculating.

The weight-or-space debate thus has an unspectacular answer: weight, with three honest exceptions. That is not a surrender to the scale, but what works in practice.