Two Arco plates are never the same, and the difference is not a flaw that slipped through. It is the material doing exactly what it was asked to do.
A reactive glaze is one that contains more than one thing that wants to happen at the top of the firing. In a plain glaze the ingredients melt together into a single even glass, and the colour is whatever oxide was stirred in, spread flat. In a reactive glaze there are minerals that separate as they melt, that crystallise as they cool, or that run where the coat is thicker. The kiln decides how far each of those goes, and the kiln is not quite the same twice.
Arco is high-fired stoneware, taken to around 1,250°C. At that temperature the glaze is fully molten for close to an hour, and during that hour it moves. Where the plate is flat the coat stays even. Where the glaze runs off the rim it thins, and the darker body underneath shows through as a soft line. That line is called breaking. On a Moss plate it reads as a rim a shade closer to bronze than the well. On Slate it is almost black.
Pooling is the opposite. Where the form turns inward, in the well of a low bowl or at the foot of a rim, the glaze collects and thickens, and a thick coat of a reactive glaze goes glassier and deeper in colour than a thin one. Tip an Arco low bowl to the window and you will see the pool at the bottom, a small lake a tone darker than the sides.
Speckle is the third thing to look for. The stoneware body carries iron, and at high temperature some of it burns through the glaze as pinpoint flecks. A plate fired near the door of the kiln, where the atmosphere is slightly different, will show more of it than a plate fired at the back. We do not sort by this. A set of six Arco dinner plates will have a few that are more freckled than the others, and the table looks better for it.
Glaze variation is the record of the firing. Sanding it out would be like planing the grain off a table.
There are marks that are faults, and it helps to know them apart. A crawl is a bald patch where the glaze pulled back from the body and left it bare; it is a sign of dust or oil on the biscuit and it is not accepted. A pinhole is a tiny crater that goes through to the body and would trap water. A crack is a crack. Every Arco piece is checked for these after cooling, held to the light and sounded, and anything that fails is broken down and returns to the clay.
What is accepted, and what you should expect: a rim that varies in tone around its circumference, a well that is darker than the walls, a scatter of iron flecks, and a surface that is faintly uneven under the fingertip because the glaze has moved. All three colourways show it. Clay shows it least, because the glaze is paler and the contrast with the body is softer. Slate shows it most.
The practical consequences are small. Reactive glazes are as hard-wearing as any other stoneware glaze, so Arco goes in the dishwasher and the oven to 200°C. Cutlery marks, the grey lines a knife leaves on a matte surface, lift with a paste of bicarbonate of soda and water. Do not use abrasive powders, which dull the glaze without removing anything.
If you are buying a set and want it to read as a set, buy it from one firing. We list Arco by batch, and pieces from the same batch share a kiln and a day. If you are adding to a set later, expect the new pieces to be siblings rather than twins. Put them on the table together for a month and you will stop being able to tell.