A fired dimension is a range, not a point
A drawing shows a dimension as a single figure. A fired ceramic body delivers a dimension as a range around that figure. The difference is not sloppiness; it is a property of the material and of the process that forms it, and every manufacturer of ceramic ware works inside some range or other.
What buyers are really choosing between is not a supplier with no variation and a supplier with variation. It is a supplier that knows its range, controls it and states it, and a supplier that does not. A tolerance that has never been written down is still present in the goods; it is simply unmanaged, and it is discovered on site instead of on paper.
What the kiln does to size
Ceramic ware is formed while it is still soft and is then fired to make it hard. During firing two things happen that both affect dimensions. The body shrinks as the material densifies, and it can distort as it does so, because a shape that is standing unsupported at temperature is affected by its own weight and by uneven heat.
Shrinkage is predictable in a plant that controls its process and unpredictable in one that does not. Distortion is harder to eliminate entirely, particularly across flat surfaces and long spans, and it is the reason a fired dimension can move more in one direction than another. A body that is held well over a short dimension can still show movement over a long one.
This is a process property rather than a defect, and it is why a factory that fires its own bodies understands its tolerance better than one that buys finished parts. Our bodies are fired in our own tunnel kiln at Chaozhou, with casting, glazing, firing, QC and export in the same plant: a factory established in 2021, covering 51,000 m², employing 285 people. Every model, across all 89 codes, is vitreous china.
Which dimensions are held tight
Not every figure on a drawing needs the same treatment. Some dimensions sit at an interface with something the factory does not control, and those are the ones that have to be held hardest.
- Fixing dimensions, because they have to line up with a frame, a seat or a set of fittings supplied separately.
- The waste connection position, because it has to meet drainage that is already built into the floor or the wall.
- The rim and seating face, because the seat is produced to sit on it and a poor fit is visible on day one.
- Any figure the project has already set out from, such as a service void or a tiling line already established.
The common thread is that these dimensions are shared with another party. A variation there does not stay inside the factory; it becomes somebody else’s problem, and that is why interface figures are the ones to specify most carefully.
Which dimensions are allowed to move
Other figures are more forgiving, and treating them as though they were critical creates its own problems. A buyer who demands an absolute figure on a free dimension will be quoted either an unrealistic promise or an unrealistic price.
- Overall silhouette, where a small change is invisible and where the shape is not asked to meet anything.
- Cosmetic surface figures, where appearance is judged by eye at a distance rather than measured against a datum.
- Free edges, which do not connect to another component and therefore do not pass variation onward.
The distinction between the two groups belongs in the order. A specification that applies one tolerance to every figure on a drawing is asking the factory to hold a cosmetic dimension as tightly as a fixing dimension, which is work that is paid for and rarely needed.
How to write a tolerance into an order
A tolerance is only useful if it names three things: the figure, how it may vary, and what happens if it does. Written that way, it can be checked on arrival without argument.
- The figure itself, and the view it is taken from, so both sides measure the same thing.
- The variation allowed, stated as a range around the figure rather than as a single value.
- Which figures are critical and which are free, so effort is spent where it matters.
- The method of checking, including where the piece is measured and with what, so a verdict does not depend on who is holding the tape.
- What happens if a piece falls outside: rejection, replacement or acceptance, agreed before the goods ship.
The last item is the one buyers most often leave out, and it is the one that decides whether a tolerance has any force. A stated range with no consequence is a description; a stated range with an agreed action is a requirement.
What to do when a delivered piece disagrees
Disagreements about dimensions usually start with two people measuring different things. Before any claim is made, confirm which figure is being tested, which view it comes from, and which method was used. Most cases resolve at that step, because the tolerance and the measurement were never aligned in the first place.
Where the measurement stands and the piece is genuinely outside the agreed range, the answer is whatever the order said would happen. That is why the consequence belongs in the order rather than in a later conversation. Where the order is silent, the discussion becomes a negotiation about goodwill rather than a check against a specification.
It also helps to measure a sample of pieces rather than one. A single figure out of a whole delivery is evidence about one piece. A set of figures across a delivery is evidence about the run, and the run is what the specification was written about. Our inspection checks before packing are designed to catch departures at that stage, while the goods are still at the plant.
Why tolerances matter most at the joints
Most of the trouble that variation causes appears where two components meet. A pan and a seat, a body and a frame, a fixture and a service void: at each of these, the variation of one part is added to the variation of the other, and the result is what the installer sees.
That is a reason to think about tolerance as a shared question rather than a local one. Unless one party holds both halves of an interface, the sensible approach is to state the figures at the interface and let each side hold its own part of them. Writing the interface figures down is what stops two compliant components from arriving as an incompatible pair.
Read a tolerance as an instruction about where the variation is allowed to go. Written into the order at the figures that matter, it turns a discussion that would otherwise happen on site into a check that happens on paper.
Frequently asked questions
Why do fired ceramic dimensions vary at all?
Because the body shrinks and can distort during firing. A shape standing unsupported at temperature responds to its own weight and to uneven heat, so a fired dimension lands within a range rather than exactly on the figure on the drawing. This is a property of the process, not a fault in the goods.
Which dimensions should I hold tight in a specification?
The ones that meet another component: fixing dimensions, the waste connection position, the rim and seating face, and any figure the project has already set out from. These are shared with another party, so variation at them does not stay inside the factory. Cosmetic figures can usually be allowed more room.
Should every dimension carry the same tolerance?
No. Applying one tolerance to every figure on a drawing asks the factory to hold a cosmetic dimension as tightly as a fixing dimension. That is work the buyer pays for and rarely needs. Split the figures into critical and free, and specify only the critical ones as critical.
How should a tolerance be written on an order?
Name the figure and the view it comes from, state the variation as a range, say which figures are critical, describe how the piece will be measured, and agree what happens if a piece falls outside. The last item is what gives a tolerance force; without it the range is a description rather than a requirement.
What if a delivered piece is outside the stated range?
First confirm that both sides measured the same figure, from the same view, by the same method; most disagreements end there. If the piece is genuinely outside the range, the outcome is whatever the order agreed in advance. Where the order is silent, the discussion becomes a negotiation rather than a check.
Does warpage affect installation?
It can, particularly where a flat face has to meet a flat surface or where the piece sits against a rigid frame. That is why variation matters most at the points where two components meet, and why interface figures should be stated by both parties rather than assumed by one of them.
Related on this site
Documents and certificates
Related reading
- Reading a sanitaryware specification sheet
- What vitreous china describes
- How Colour Is Applied: Glaze and Firing