andros Posted January 25, 2021 Report Share Posted January 25, 2021 Recently I’ve made some nerikomi earthenware pieces using various stains, but I have an annoing issue when applying the clear glaze, that is some colours passes a light yellow colour to the glaze. The unfired glaze takes from the bisque body a little yellowish “halo” that remains visible also after firing. This happen with the yellow stain (obviously…) but also with the black (surprisingly). Being in Italy I don’t use Mason Stains, but they are stains, and this issue is pretty strange… How can a fired body pass it’s colour to the glaze? May do this due to the fact that earthenware is fired to a temperature too low to “fix” the stain’s colurs to the clay body? Link to comment Share on other sites More sharing options...
JohnnyK Posted January 25, 2021 Report Share Posted January 25, 2021 If there is iron in the glaze, that might be the problem...here is an example of some pieces that are an example...The unit on the right has been fired without glaze and the one on the left has been fired with clear glaze. The initial coloring was red iron oxide. I was told that the RIO reacted with the clear when it was fired to ^06, thus causing the mottled coloring...there is the possibility that something similar is happening to you... Link to comment Share on other sites More sharing options...
Min Posted January 25, 2021 Report Share Posted January 25, 2021 3 hours ago, andros said: The unfired glaze takes from the bisque body a little yellowish “halo” that remains visible also after firing. Sounds like soluble salts. Can you post a picture? Link to comment Share on other sites More sharing options...
andros Posted January 26, 2021 Author Report Share Posted January 26, 2021 Here I have an unfired piece (neglect the poor glazed surface, I'm going to repeat the glazing...) and a fired test piece. In the fired piece the yellow colour is mostly visible in the corners, that should have been white with litle green "veins". In the unfired piece the colour is visible a little everywhere... Link to comment Share on other sites More sharing options...
Bill Kielb Posted January 26, 2021 Report Share Posted January 26, 2021 Just to eliminate the obvious first have you fired a palette of your stains unglazed as well as glazed to various temperatures on a strip of your claybody to see what temperatures the various colors are good to with whatever variety of clear overglazes you use. Some colors burnout or change more so than others. Without this type of test it is often hard to know the limits of each color / temperature / glaze combination on a given claybody. Link to comment Share on other sites More sharing options...
andros Posted January 26, 2021 Author Report Share Posted January 26, 2021 Unfortunately I've limited possibility to change firing temperature. Since I don't have my kiln I must rely on shops that fire my stuff for me, and they use standard authomatic firing programs, probably already preset by the manufacturer. These programs works good for "traditional" earthenware and glazes, but I can't experience anything different.. I'm looking for little more experienced craftsmen here in my city to ask about the possibility to use of their kiln, but until then I think I will have no choice but to exclude some stains... I was just wondering the possible cause. Link to comment Share on other sites More sharing options...
Sorcery Posted January 26, 2021 Report Share Posted January 26, 2021 Have you ever fired them unglazed? Perhaps it's not the glaze. Some Stains are Body Stains, some are not. Don't know if that applies here with your materials. Sorce Link to comment Share on other sites More sharing options...
andros Posted January 26, 2021 Author Report Share Posted January 26, 2021 36 minutes ago, Sorcery said: Have you ever fired them unglazed? I'm not sure to understand what do you mean, sorry... I mix the stains to many pieces of "fresh" clay, then I put them together in some way to create the colored pattern. Before applying the glaze I make a first bisque firing, then I apply the clear glaze and fire a second time. Both firings are made to the same temperature: 980°C (1800°F). The bisque don't shows anything strange , but it's hard to say, because the body is already colored and I don't have any reference to judge if something should be different. What is sure is that if I remove the unfired glaze, I see that the white parts of the body are still white, that is: the yellow colour evidently born in the colored parts and spreads to other parts through the glaze.... Link to comment Share on other sites More sharing options...
PeterH Posted January 26, 2021 Report Share Posted January 26, 2021 On 1/25/2021 at 2:09 PM, andros said: ... Being in Italy I don’t use Mason Stains, but they are stains, and this issue is pretty strange… How can a fired body pass it’s colour to the glaze? ... Perhaps it's time to ask what the stains are (manufacturers name, stain name, url to an advert or suppliers description). Perhaps somebody can unearth something about their constituents, if only with google translate. PS Might be worth checking that you cannot lift any colour off a bisqued piece with a damp paper towel. Link to comment Share on other sites More sharing options...
Min Posted January 26, 2021 Report Share Posted January 26, 2021 Looks like soluble salts/sulphates in your images. Need to figure out where they are coming from, the clay, your water supply or the stains. Can you get info from a safety data sheet on what is in the claybody? A tiny bit of barium carbonate is added to clay bodies that have efflorescence / soluble salts/sulphates to prevent this. If you can find this information out it could rule out the clay being the culprit. Link to comment Share on other sites More sharing options...
andros Posted January 27, 2021 Author Report Share Posted January 27, 2021 I'm asking a datasheet to the reseller. He don't indicate online the brand \ name of the stains he sells, but I think they are produced by Colorobbia (the main italian glazes\stain\clay producer) which does not gives any detailed information online about stains since here in Italy stains are still intended almost only for industrial use. Only in the last 2-3 years I started to see stains in the resellers catalogs (for hobbyists\small craftsmans use), but with very limited informations ... as soon as I have some info I will share them. For now thank you for your precious support !! Link to comment Share on other sites More sharing options...
PeterH Posted January 27, 2021 Report Share Posted January 27, 2021 http://www.colorobbia.it/lang1/glazes_and_colours_colorobbia_specialties.html 2 hours ago, andros said: I'm asking a datasheet to the reseller. He don't indicate online the brand \ name of the stains he sells, but I think they are produced by Colorobbia (the main italian glazes\stain\clay producer) which does not gives any detailed information online about stains since here in Italy stains are still intended almost only for industrial use. Only in the last 2-3 years I started to see stains in the resellers catalogs (for hobbyists\small craftsmans use), but with very limited informations ... as soon as I have some info I will share them. For now thank you for your precious support !! @androsLooking at Colorobbia's "general catalogue" at http://www.colorobbia.it/lang1/files/Cat.Gen.COLORI_2010.pdf I can only see "decorating colors" in their CSC & CSM, and lead-free CAV ranges (p16). These are described elsewhere in http://www.colorobbia.it/lang1/glazes_and_colours_colorobbia_specialties.html as COLOURS These leaded or lead-free, under-glaze (CSC) or over-glaze (CSM) colours are obtained by mixing pigments with fluxes and can be used directly on the bisque or on objects that are already glazed. Applied by brush, sponge, pad, spray gun or screen printing. The lead-free ventilated CAV series allows for dry mixing of colours and is suitable for colouring white glazes or neutral transparent frits by means of simple dispersion. Are these what you are using, and can you find any product code (e.g. CSM43) on your packaging? PS To the experts. I've found Colorobbia's name on a couple of references to spinels. Still looking for anything that says they sell stains.https://patents.google.com/patent/ES2711378B2/en?q=spinel&assignee=Colorobbia&oq=Colorobbia+spinelhttp://www.ipconsortium.eu/sief.php?ein=269-047-4&idi= Link to comment Share on other sites More sharing options...
andros Posted January 27, 2021 Author Report Share Posted January 27, 2021 That colours are pawder underglazes\overglazes intended for majolica, and are basically like "US style" underglazes but wirhiut hardening (and I mean hardness in the not fired stage) and suspending\brushability agents, since they are used as "watercolors". There's no mention of stains the Colorobbia catalog. The only mention of stains in the Colorobbia website is here: http://www.colorobbiaitalia.it/lang1/prodotti_colorobbia_italia.html This is a section intended for industries. No catalog nor detailed information unless explicitly requested to the sales department, who takes into account your request only if you are a real potential business partner... Anyway the re-seller just gave me an unhoped fast response!! No datasheet but this message: "from the photos it is clear what the problem is. These are soluble salts of Vanadium and Chromium, present in an infinitesimal part in the clays and not fixed by the firing. In the presence of clear glazes with an alkaline pH, a process of solubilization of these salts and the coloring of the "glass" itself take place. Remedy: use an anti-yellow agent in the glaze or lower the pH of the glaze by frequently changing the deposition water or adding small quantities of citric acid. The stains do not hit anything, I confirm instead that the high thickness of the glaze enhances the problem. " I don't know what this "anti-yellow agent" could be, but his answer makes sense to me, even if I saw this problem only with colored clays... but maybe I remember wrong, I haven't used clear glazes over uncolored clay for a while.... Link to comment Share on other sites More sharing options...
Min Posted January 27, 2021 Report Share Posted January 27, 2021 3 hours ago, andros said: but maybe I remember wrong, I haven't used clear glazes over uncolored clay for a while.... Time for some testing of the variables to determine where the salts are from. Variable 1 is the clay itself, variable 2 are the stains, variable 3 is your water. I'ld start by emailing Colorobbia and include the reply from your re-seller. I wouldn't have thought stains would release soluble salts, see what Colorobbia says about this. If Colorobbia agrees that it could be soluble salts from their stains then you would need to test each stain by itself to determine which ones are causing this. Test 1: using clay fresh out the bag make 3 test samples with the unstained clay, glaze one with the glaze you already have made up. Glaze a second test sample with glaze made with distilled water. Third test tile don't glaze. Fire them all as you normally do. (Since your bisque and glaze temperature is the same the unglazed tile need only be fired once.) Barium carbonate is added to clays that have problems with soluble salt scumming, just a very small amount is needed. The barium carbonate binds the salts within the body so they are not able to migrate to the surface of the clay. Using an acid, such as vinegar can wash off surface salts but repeated wetting of the clay can bring salts to the surface again, this isn't a permanent fix. Post the name of the clay you are using, maybe someone can track down the safety data sheet for it and see if barium is added to the clay or not. . Link to comment Share on other sites More sharing options...
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