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Nope, boron is high in both, (yellow box in the unity formula below the recipes) 0.30 is usually around the top end for a durable cone 6 glaze. Could be better but if I reduce the boron and add another flux it's going to be easier to just start with a better recipe. Using your Ferro frit 4108 it's pretty much identical chemistry wise to the original.(potassium a titch less and sodium a titch more but combined they are equal) I would add 2 bentonite to this version if you try it.

I seem to remember you have gerstley borate? Recipe below using it to supply the boron. Sodium is lower and the g.b. adds a titch of magnesium. Silica, alumina, calcium, boron are the same. Should meIt okay but test it first. If you don't have gerstley which boron containing frits do you have?

Since you are selling pots I would stop referring to yourself as a hobby potter. You are a potter. Call yourself as working part time at it if you like but to me the term hobby potter doesn't really encompass selling work on the scale you have described. Being a hobby potter is wonderful, I hope to get there someday, but from a business perspective that's not how I would market yourself.

I don't know how thin you cast your pieces but I found I needed a hotter bisque than 980C (approx cone 07) or the bisque absorbed too much glaze and it cracked. This has 8% Mason Best Black stain in the black slip, no glaze on the exterior, glaze fired to cone 6 (approx 1220C). This one doesn't have black stain on the bottom but others I've done with black slip on the bottoms and absolutely no sticking to the kiln shelves (plucking). (I wouldn't recommend not glazing the outside unless you really test them for dunting)

As a side note, Laguna lists the absorption for that clay at 3% at cone 5. Since it looks like you are making functional pots I'ld stick with cone 6 to try and get that absorption figure down a bit otherwise you can have leaky pots (with or without glaze). I'ld strongly suggest using some witness cones and go by what they are telling you than just using the bar in the sitter. I'ld also run your own absorption tests since the data from clay companies comes with some wiggle room insofar as absorption figures. (info on how to that here if you need it, fairly straightforward)

Same principle behind the Bailey machines, after the mixing chamber the clay goes through screens which extrude spaghetti like extrusions which are de-aired then clay goes along to the final auger to be extruded. Different method than the Peter Puggers which de-air the mass of clay in the chamber.

Looks a fair bit like the Mayco Jungle Gems. (image from their website)

I often do this, no problems.

Please post the approximate location and price you are asking.

Interesting thoughts here. To rule in or out the microbes or vacuum theory I'ld try taking the scrap, misting it lightly with water then hand wedge it and see how plastic it is. If it's plastic then it would point to the vacuum being the issue. I can't see how the microbes would decrease in the pugger unless it was being pugged for so long that the clay gets hot enough to kill off the microbes. Doubt it would get hot enough though.

If the earthenware is for functional work many raku glazes wouldn't be suitable for food surfaces due to the lack of durability of the glaze and/or the amount of colourants used. If it's a basic clear or white raku glaze it might be fine but without knowing the recipe it would be hard to say.

Fellow in the video is using ceramic stains, if your product can be used like this then it sounds like "universal colour" is just the terminology being used in Denmark for stains. I don't know if you are using lowfire or mid or highfire slip, the higher you fire the darker the black should become but depending on what the stain/colour is made from it could increase the fluxing of the slip. Using a cookie (waste thin slab of clay) under the first glaze firing is a good idea like Rockhopper suggested, brush a quick coat of kiln wash on it if you want to be extra safe. In the slight chance it sti

Could you send a link of what this product is? I can't recall this product being discussed here before. It's probably fine, but just in case it would be good to have more info before saying for sure. Welcome to the forum.

The Mason Best Black stain you are using contains iron, cobalt and chrome, the blue is from the cobalt where the glaze is thicker where it's pooled. That shade of green is from the cobalt plus chrome and iron. Look for a fluid glaze recipe, that doesn't contain zinc (zinc makes chrome turn brown), and run some tests. Alumina is one of the things that makes a stiff glaze, to make a glaze runnier reduce the alumina in the recipe. Easiest way to do this is reduce the clay content in the recipe, usually this will be EPK.

Just an idea but if you know the size of the finished slab (roughly) and you know the size of a bag of clay you could do the math and go by the volume of slab and clay bag. Laguna box of clay is 12X12X6 so one bag is approx 12X6X6 which has a volume of 430 cubic inches. Your 30X30X0.25" slab has a volume of 225 cubic inches. Since the bags of clay aren't quite as big as the box and to make the math easier I'm rounding the bag of clay down to 400 cubic inches. 225/400 rounds down to 9/16th of a bag of clay, or just over 1/2 a bag.

This is called crazing in ceramics, when seen on pots taken just from the kiln it's usually caused because the glaze doesn't fit the clay it was used on. What's happening is the glaze is shrinking more than the clay during the time the kiln is cooling down. The glaze has to "stretch" to cover the clay, this stretching causes the glaze to "tear" / craze. This can work if the kiln thermocouple(s) used to measure the temperature is accurate but the temperature they measure tends to drift as the thermocouple wears. Even brand new there can be differences between the temperatures thermocou

We have been considering moving for several years, since 2021 is still an unknown it might be time to pack up my studio/workshop and go for it. Thought of how much equipment and supplies etc I have is really quite daunting so I might just procrastinate a bit longer. 2020 BUH-BYE! (kind of looks like a trimming chuck so I figured I can get away with posting this)

Like Bill has said there are two main issues you bring up, crazing and durability. I don't use the term "food safe" as I have never seen a scientific definition of what this actually is. Crazing is okay if the claybody is vitrified and good kitchen hygiene is practiced, the crazing does weaken the pot though, whether it's enough to be relevant is up for debate. If clay and glaze are both fired to maturity then crazing is often the result of the glaze and clay not fitting each other. In effect the glaze is too small for the pot so as it cools in the kiln it shrinks more than the clay and h

A quick Google search brought up a few candle making supply places in Egypt, heres one https://www.ubuy.com.eg/en/search/?q=candle+making&node_id=1862that sells bags of flaked soy wax. Soy wax doesn't smell nearly as bad as paraffin wax when burning off in the kiln.

I find that a jiggling pressure like what the dentist does to your cheek when they inject anaesthetic (sorry best description I can think of) helps the two pieces grab onto each other. Did you see any slip squeeze out along the edges after joining the pieces? Welcome to the forum.

In ceramics the oxides we predominately use for colourants are some of the transition metals (plus occasionally but not often some of the rare earth metals). Transition metals commonly used are cobalt, copper, iron, chrome, manganese, titanium (works as an opacifier, glaze variegator and colourant) and nickel. Cadmium and vanadium are also a transition metals but they aren't used in oxide form but rather incorporated into stains to make them safer to use. Zirconium is also a transition metal, it's used to opacify a glaze. Some of the colouring oxides can be used by mixing with water and

Glad you found a schedule that works! I wonder if you might be able to skip the hold at the middle drop temperature and go from the end of the top hold to 1900F and hold there. If the glaze is still fluid enough at 1900F to heal pinholes this (in theory) should work.

Going forward see if leaving the wax resist dry for longer helps avoid this. If you are making your own black resist you could try another brand of the resist itself, some definitely work better than others.

If the porcelain has been fired to maturity it won't stain/discolour.

I would hazard a guess that the Aardvark Soldate 60 might have more flux than the Cavener version of it, she fires to cone 2 and notes her version of Soldate 60 easily goes above cone 10. Is it Greenstripe fireclay that you like in the Aardvark Soldate 60? It's very straightforward to test absorption, link here on how to do that and slump testing also.