Tuesday, 31 December 2019

Buff white - basal white with underglaze decoration

Class results continue - 2nd firing, buff clays with a white dip.

When wet, the PB103 and the Buff clay have a noticeable colour difference. The buff clay looks like it resembles the 'earthenware' clays in the museums more. I guess, brownish clay (with some fe-oxide) content seems more natural to me. I've already posted my first experiment with the buff clay effect on glaze colour and noted the curious tendency of the buff to bloom with certain glazes. The items described in this post, were fired in the second batch but decoration commenced while batch one was still in the kiln.

If you've been following my blog, you'll know I started experimenting with underglazing in May / June 2011. I spent many years visiting Glaze It Studio and using their commercial bisque, underglaze and firing. The team there are lovely! Siegel (hope I spelled that right) still welcomes me when I drop in once or twice a year. Many years ago, she gave me glazes to take home so I could work on my items in peace and have been more than generous over the many years. I have, unfortunately, moved so attending their lovely studio is very challenging. I have tried a couple of similar studios in my new home town but the atmosphere is just not the same. The time and dedication it'd take to build a similar relationship with a new company is beyond me because I just want to be able to create my art.

Enter ClayMake Studio! This started as a way to broaden my ceramic skills as I'd been introduced to throwing at Pennsic by Master Simon. Interested in learning more I've signed up for a single session class and a longer 5 week wheel work class. As a result of these classes, I have plentiful bisque to work with and I can indulge in underglazing again. I do need to learn some new parameters as ClayMake fire to stoneware temperatures which, I believe, may be a lot higher than GlazeIt fire at. I'll still be returning to GlazeIt on occasion as I have yet to complete my tile project (ongoing since 2013). There are a few tiles I haven't posted about but I plan on firing the last one and doing a couple of posts about that project. It's been an interesting journey.

Fragment, 14th–15th century, Egypt, Kus, Stonepaste; underglaze blue and black; transparent, colorless glaze. Met Museum item number: 07.238.19

Anyway, that was an epic background introduction to this new project. I've previously only used / had access to white commercial bisque. This has made creating my designs really easy as many of my medieval references are underglaze on a white lead glaze background. The image above is an excellent example. The brownish nature of the clay can be seen on the edges of the fragment and the white basal colour fragments with curved edges suggesting a silicious nature. (Side note: I've been meaning to develop this fragment into a full design for a couple of years now. I've only just realised the main checkerboard is probably a tree with blue fruit globes and a blue bird while the angular design in the bottom right is a building or fence perhaps.)

Anyway, I've been able to do away with the background colour and concentrate on the decorative motifs in the past due to the bisque I used. Given the wet colour of the buff clay, I was concerned that if I continued to use buff, it'd change my underglaze colours too much. This experiment was proposed as a solution of the colour of the clay. Similar to some majolica pieces, these cups have been dipped in a white background glaze before the underglaze has been applied. (I don't remember which glaze I used, I may have screwed up and used Titanium white instead of satin white!).

Pre-firing, basal white glaze with Redwood, Citris / Saffron and Turquoise with three layers thick.

The cups were dipped in a white background glaze and then transported to my house for underglazing about a week later. As a result of this delay, all of the moisture has evaporated leaving a very very powdery basal glaze behind. I was able to easily smooth any drips in the basal glaze by rubbing a finger lightly over it. I am a little concerned that the underglaze brushes, if too hard, have lifted some of the basal layer and I may end up with specks in the underglaze colours. The cup on the left has a black outline and detail under the three coats of underglaze colour. I'm curious to see if that comes through or not. I will have to do a proper multi-coat tile experiment with these glazes because the reference tile I've currently got really doesn't show if the colour changes during firing are just due to the thinness of the glaze coat or not.

Test tiles for Buff clay, left = Titanium, right = plain with clear overglaze.
Colours left to right : Row 1. summer blue, electric blue, cobalt blue, turquoise, peacock, jade. Row 2. Redwood, Crimson, Saffron, Citris, Leaf, Brunswick. Row 3. 28A (glazeit colour), Black oxide, Light Jade. Row 4. 28 original (glaze it colour for my bathroom tiles).

I'm in two minds if I should cover everything with a clear overglaze or just hope the basal layer has sufficient silica to make the designs shine. I'm not sure how it'd look if I did dip it in Satin White as I planned on doing. The test tile on the left shows the titanium has sufficient silica to make a pleasing shine to the glaze. The titanium seems to react with the underglazes, modifying their colours and making them more speckled. I chose to use colours that seemed to work on both tiles in the hope they'd stay true. I have a fourth tile and will conduct a test with the clear overglaze with it and leave these as they are.

I used hummingbirds as a motif because if the underglaze does turnout to be titanium white, the speckle would still look good. If the streakiness seen on the previous underglaze experiment was an interaction between the clear overglaze and the underglaze rather than an overglaze application error, the humming bird motif won’t be badly impacted.

Sunday, 29 December 2019

PB103 Iznik vase - experimenting with new and old underglazes at high temperatures

Item four in the Christmas firing was a small PB103 Iznik vase. I was experimenting with thin closed forms when I made the vase. It was reasonably pleasing to hold to I kept it and fired it to bisque. Once I purchased a new set of underglazes, it seemed like a good candidate to trial the colours at stoneware temperature. The following are the colours I had available. These are presented on a buff coloured tile so the PB103 expression is likely to be different.

L-R: R1. summer blue, electric blue, cobalt blue, turquoise, peacock, jade. R2. Redwood, Crimson, Saffron, Citris, Leaf, Brunswick. R3. 28A, Black oxide, Light Jade. R4. 28 original.

Not knowing how the colours would turn out, I selected Crimson, Turquoise and Cobalt blue for my design. I know blue is stable at high temperatures (see the Blue on White essays) so I figured those elements would probably remain. The temperature range on the crimson suggested it'd probably burn away so I used it for non-essential items. These were the small swirls at the base of the vase, the flower insides and the dots around the petals.

Close examination of the flowers show a very pale pick remains. I may use crimson in the future if I'm aiming for a pale pink as I'm told pink and purples are difficult to deliberately achieve. If I want a bright red, I think redwood may be the way to go. I'll have to make a PB103 tile and test the glazes on it to see. On the buff clay it seems to be more orange and less deep red than I expected. It will require a test though, because the buff clay contains something that reacts with the glaze, creating blushing near where some specific glazes are applied.

Saturday, 28 December 2019

Buff blue - glaze dipping to understand difference from PB103

Glazing attempt 2 - blue!

My first attempt at glazing was a single session to experiment with colour combinations and basic dip glazing techniques as dip glazing was something I'd never tried before. Those experiments were moderately successful with the blues being my favourites followed by the aubergene over green combination. For medieval ceramics through, the purple colour isn't a common one so I thought I should stick with basic colours to see if using buff clay changes much.

The first experiment was conducted on what could be my favourite little cup shape. It unfortunately developed a small crack in the base and one on the handle due to uneven drying. I hoped a solid glaze could plug any potential gaps and strengthen the cup. I had intended to put a clear glass over the matte blue however the cup slipped while I was dipping it so it absorbed too much water from the glaze and was too damp. I didn't want to add more moisture with a second layer of glaze as I was worried it wouldn't be able to be fired. Still happy with the shape as I love the little thumb divoty thing and the handle is a very pleasing size for me.
The glaze appears to have strengthened the cup and this one it nicely watertight and the handle can hold the weight of the cup full of liquid.

Experiment the second. This smallish milk jug was dipped in titanium white in the base and matte blue on the top. The inside was sealed with a transparent gloss glaze. Blue dots of 28B underglaze was added for visual appeal and to see if the older underglaze could withstand the stoneware temperatures (~1200). I am in love with this glaze blend and plan on creating a whole set of house cups for myself in this pattern.

Experiment is a success with the blue withstanding the higher firing temperatures and the buff clay body is not skewing the shades blue significantly.

Interestingly, the buff clay appears to bloom when proximal to certain types of glaze. There doesn't seem to be a significant colour change on the foot of the milk jug however there is bloom on the foot of the blue cup. The bloom is easier to see on the neck of the quatrefoil cup resulting in a pleasing reddish-brown tone. This has to be a result of a slight degassing from certain glazes interacting with the iron in the clay body as the same glazes on the PB103 do not produce this result.

Friday, 27 December 2019

PB103 quatrefoil cup - still chasing the grail

My first quest in my recent ceramic class was to make a quatrefoil cup. I threw three but squished two of them because the proportions just didn't look right. My final one still isn't perfect but it was the best of the lot and the shape visually pleases me.

So what is a quatrefoil cup? A quatrefoil cup has a rotund body, which cinches in slightly then flates out. The lip is pinched so as to form four lobes. The lobes take up slightly less than half the cup. There are a number of quatrifoil and cinquifoil extant items available in museums. My favourite is posted at the bottom of this entry because the Swiss National Museum doesn't create independent linkable pages for their items.

Quatrefoil Cup. 9cm tall. 15th century, Cheapside, London.

My first attempt at a quatrefoil cup was my third ever thrown item. I threw it at Pennsic in 2019 and it's details can be found here. My next attempt at a quatrefoil occured during day 1 of a 5 session class. I'm still learning about how to determine the stability of clay and how to intuitively feel when it's been pulled enough. I started using PB103 which is a basic white clay. It's reasonably dense and can sustain a lot of height on reasonably thin walls.

There are three main things wrong with my quatrefoil. First, height: The original is 10cm wide and 9cm tall. My cup is taller than it is wide, resulting in more vase overtones. Second ratio: the original had the quatrefoil lip flaring out from just over a half cup. Mine is about the top third of the item creating a 60 degree angle rather than an 80 degree angle for the lip. Finally glaze: I used a matte green glaze on the base of my cup and a clear internal & over the lip glaze. I should have reversed this, glazing the inside and lip green and then sealing the lot with a clear glaze over everything.

Interestingly, the colour variant in the glaze of my cup is due to the firing process rather than a deliberate attempt. I'm not sure if this a slightly temperature change or a slight oxidisation change both could be caused by the bulbous nature of the cup.

Breiter Henkel mit seitlich eingedrückten Rundflächen; enger Hals. Herstellung: Zürich. 1400 - 1500. Herkunft: Zürich (ZH), Haus zum Johanniter. 23,4 cm. Ton, glasiert. (Broad handle with indented round surfaces; tight neck. Production: Zurich. 1400 - 1500. Origin: Zurich (ZH), Haus zum Johanniter. 23.4 cm. Clay, glazed.) Swiss National Museum. Item no. LM-24622 or possibly DIG-4681

+1 to throwing skills

I believe I've achieved a +1 in pottery skills in the last couple of months. I've just completed a 5 week (three hours a week) course at ClayMake Studio and my skill at making things has come along significantly. Prior to this course, I made a bowl at Pennsic in  2017, a cup and a bowl in 2019 and 5 "bowls" in a oneshot single lesson session in October which I glazed in early November. 
Enthused, and city based for the time being, I signed up for a 5 week throwing course with the aim of being able to throw a satisfactory cup in a deliberate manner. i.e. I want to be able to throw a set of similar looking cups whenever I want. As a side mission, I'd like to understand clay and the strength of the form well enough to be able to throw a properly balance quatrefoil cup.

As this class went for 5 week, 4 throwing, 1 glazing. I spent 1 class throwing items in PB103 which is white clay and then shifted to a buff clay for the next session as it was closer in colour to the earthenware I was trying to replicate. I spent the third and forth sessions trimming pots and learning to make handles and for the fifth session I glazed some of my creations.

I had a lot of pots to glaze! Due to my interest in underglazing, I decided to do some experiments with underglaze rather than just dip all my items like I did for the oneshot class. Below is my experimental colour test tile which was fired with the first 6 items. The first six were experiments to see if the underglaze held up and if my cups would make it through stoneware firing. I only underglazed three of these and accepted they might not come out of the kiln well. I figured I had a pretty good shot with the black oxide and the other colours may or may not work. They were ready for collection on the 27th!

All my items are at various stages of glazed so I'll post about them and update this entry as they develop.

PB103 mustard - my first attempt at decorating a mostard jar (pending firing)

Buff greens - middle-eastern / sultanabad - Part 1, Part 2, Part 3

Handles - why summer is BAD