250grams Contem UG pot
= 175 ml
= 5.9 FlOz
PLEASE NOTE THESE PICTURES SHOW THE UG’s AFTER A TOP GLOSSY GLAZE HAS BEEN PUT ON AND FIRED
These are stain based underglaze brush on’s that hold their colour integrity over a wide range of temperatures from 1000 °C to 1250 °C.
Without a top transparent glaze they give a matt surface which we would not consider food safe . As they do contain glaze frit they do grab in to the pores of low biscuited clay but not high biscuited ware where the porosity has been reduced. As often used in school situations we do suggest that you explain to students that the bisque needs wiping down to remove pinholes and salts and to be extremely careful with grease from fingers. Slow bisque firing can help ensure all gasses are expelled. We do not recommend applying any underglaze on to greenware - see explanation at bottom
Although people do tell us that they can apply underglazes to greenware and then bisque fire and then apply a top glaze and refire we really do not recommend it – 95+% of people do not do it this way for a good reason. And 95% of issues we come across are people applying to greenware.
During a bisque fire at 100/120 °C the free water comes off as steam – 1 gram of water creates 1.5 litres of steam and it has to come out through the pores; 300 °C you get the water which is combined with the clay coming off as steam, then approx. 550-600 °C you get carbonaceous matter and sulphur coming off as gasses and then at 750-850 °C you get the carbonates releasing their gasses.
All of this steam and gasses have to come out of the clay pores and can push out the underglaze from the pores; they also bring salts to the surface which can boil and release more gasses during the glaze fire peeling off the underglaze.
The underglaze is a thin dense layer of metal based stains which does not have the ability to reform and run back into the pores however much flux is added.
So bisque fire slowly – wipe down all the salts / powder and then underglaze and allow to dry and then add the top glaze and do the glaze firing – and all should be well.
If the UG brush on is starting to dry out we would recommend adding small amounts of UG Medium and re-mixing it all with a hand held electric mixer.
