Kimberly Young Posted March 13, 2021 Report Share Posted March 13, 2021 Hi, I throw porcelain wares that are fired to cone 10. My glazes are a semi matte, green, blue, and curry color...I have had some customers complain that after one use, the coffee has stained the cup on the inside and outside! I have done some experiments on unsold cups and haven’t seen it, but the customers sent me pictures. Anyway, what could be causing this? Should they be fired a bit hotter? thanks in advance! Link to comment Share on other sites More sharing options...
GEP Posted March 13, 2021 Report Share Posted March 13, 2021 Have you done a vitrification test? (fill with water, leave sitting on a sheet of paper for a day or two, see if paper becomes damp) The question is whether the stain is above the surface of the glaze, or getting past the glaze and into the clay. Link to comment Share on other sites More sharing options...
Kimberly Young Posted March 13, 2021 Author Report Share Posted March 13, 2021 I have done that and it is vitrified. :/ Link to comment Share on other sites More sharing options...
GEP Posted March 13, 2021 Report Share Posted March 13, 2021 5 minutes ago, Kimberly Young said: I have done that and it is vitrified. :/ That’s good news then. The glaze looks like a satin matte, which is always more likely to stain compared to a glossy. I would recommend exploring glossy liner glazes. My main glaze is a satin matte, but I line the insides of vessels with a glossy glaze. (It will still develop coffee stains after years of use, but not after one use. ) On my dinner plates, the satin matte glaze will touch food, and one customer reported that she stained it with blueberries. I’m not that worried about it, because blueberries are not something you eat everyday. But I am thinking about changing the way I glaze those plates (using glossy across the whole food surface) because of this customer’s experience. Coffee is an everyday thing for a lot of people, so at least your mugs need to be designed with more stain resistance in mind. Link to comment Share on other sites More sharing options...
Min Posted March 13, 2021 Report Share Posted March 13, 2021 7 hours ago, Kimberly Young said: My glazes are a semi matte This could be the problem. Think of a matte (or semi matte) glaze as a sheet of sandpaper, there are nooks and crannies between the grit particles on the sandpaper. When you sand a piece of wood some of the wood dust gets trapped between the grit. Same idea when using a matte glaze, tannins etc from the coffee or tea get caught in the uneven surface. It might appear smooth but there will be high and low points on the glaze surface from microcrystalline structures on the surface from the glaze materials. Same reason why matte glazes often cutlery mark. If the glaze is underfired this could also cause a "rough" surface. Have you verified the firing using cones? I'ld suggest using a gloss liner glaze, many people use a light coloured or clear liner. Link to comment Share on other sites More sharing options...
Hulk Posted March 13, 2021 Report Share Posted March 13, 2021 Hi Kimberly! Looks like crazing - cracks in the glazeCrazing (digitalfire.com) I use crazed ware in my own kitchen, however, not for sale (or gifts). fwiw, I made a commitment to liner glazing very early on, well before contemplating sales.Liner Glaze (digitalfire.com) Link to comment Share on other sites More sharing options...
Magnolia Mud Research Posted March 13, 2021 Report Share Posted March 13, 2021 8 hours ago, Kimberly Young said: Hi, I throw porcelain wares that are fired to cone 10. My glazes are a semi matte, green, blue, and curry color...I have had some customers complain that after one use, the coffee has stained the cup on the inside and outside! I have done some experiments on unsold cups and haven’t seen it, but the customers sent me pictures. Anyway, what could be causing this? Are the users washing the ware after finishing their coffee? I have coffee and tea ware that I use and the ware will have stains if the ware is not washed frequently. When washed, the ware have no "stains". This is true for ware that have crazing and those that do not have crazing. Crazing may show "stains" in the craze lines only, the stains on the surfaces between the craze lines are due to residue from the coffee/tea. LT Link to comment Share on other sites More sharing options...
liambesaw Posted March 13, 2021 Report Share Posted March 13, 2021 If the coffee is staining the OUTSIDE of the mug, this can only happen from weeping. Weeping is when liquid can travel through the clay body. I see craze lines in your glaze, it does not fit your clay body at all. These craze lines can weaken the ceramic under the glaze and cause seeping if the clay body is thin enough. Fill a mug with boiling water and put some food dye inside. Then set it on a clean sheet of computer paper for a few hours. This will tell you if you have weeping. Link to comment Share on other sites More sharing options...
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