Monday, January 26, 2015

Pictures of Pitchers! An alkaline blue glaze for cone 2 - 4, and a steam engine!

10 inch (254mm) pitcher, earthenware, alkaline copper blue glaze over a white slip that is coloured pale green with chromium oxide.
The very first of the earthenware jugs/pitchers that I made at the beginning of January were fired last week, and I have more in the kiln as I write this that are cooling after reaching their peak temperature about 9.30 yesterday morning. The first load was of only 4 jugs and some glaze tests, fired in the smaller of my two electric kilns.

Really this firing was a test, as I wanted to see how the glazes worked over the slip coating that was on the pots. I have known some glazes to actually dissolve a layer of slip, so I wanted to be sure this sort of bad behaviour would not happen. I suppose you could say that the clay of the pot is like bare skin, in fact it is often referred to as "the body", and the slip coat is a sort of underwear, and the glaze is the outer layer of clothing! The layers of glaze and slip conceal and reveal the warm red coloured body beneath it, and there can be complicated and beautiful effects as colours are veiled and modified.

Another view of the 10 inch (254mm) pitcher.
The alkaline copper glaze that I used on two of the jugs was one that I developed myself in 2012, I put something about it on my blog for September 27 2012. I designed the glaze for cone 3*, but it seems to be able to work below that temperature, and above it, however I am not sure of just how far it can go in either direction! On its own, the glaze gives a nice transparent copper blue colour, but this is easily modified when put over a coloured slip. At the time I made this glaze I was thinking of the lovely copper blue Persian pots that are friends of mine at the Otago Museum in Dunedin!

I wonder if you also have "friends" that you like to visit in a museum collection?

9 inch (228mm) pitcher, earthenware, alkaline copper blue glaze over white slip. Note that this is the same glaze as in the pitcher in the previous photo, but over a white slip this glaze gives a turquoise blue.
Being fairly high in fluxes such as sodium, the glaze will almost certainly craze over most clays, but the crazing is rather beautiful in its own way. I would use this glaze on the outside of domestic ware, but use a none crazing and more reliable glaze on the inside where it will be in contact with food and drink.

Close up of the alkaline copper blue glaze (over white slip coloured pale green with chromium oxide).

This is my Copper Blue Glaze D for Cone 2 - 4 (with technical details supplied by Insight Glaze Software).

60.00  Ferro Frit 4110   
20.00  Soda Feldspar    
7.00  Ball Clay   
5.00  Lithium Carbonate    
5.00  Silica    
3.00  Bentonite    
3.00  Copper Carbonate 

0.19  CaO 
0.20  Li2O 
0.01  MgO
0.05  K2O 
0.55  Na2O
0.25  Al2O3 
0.07  B2O3 
3.22  SiO2 
0.07  CuO 

Si:Al:  12.81
SiB:Al:  13.08
Thermal Expansion:   8.89

As an experiment I did try firing two of my alkaline copper blues to cone 10**, just to see what would happen. At cone 10 there was considerable bubbling of the glaze and some running and a shift in colour from blue to green.

Alkaline copper glaze B and D. Back row fired at cone 3 and front row fired at cone 10.
Whilst this glaze is not really usable at cone 10, I am testing glazes these days well above and below their limits, just to see what will happen. I have found one or two treasures that way. Cone 6*** glazes will sometimes be quite comfortable at Cone 10, and may even "improve" at the higher temperature by becoming more fluid. Crystalline glazes are an example of glazes that look close to being formulated for low earthenware temperature, being fired at high stoneware temperatures.

Abbot's Clear
The remaining two pitchers have on the outside a commercial clear glaze that is available in this country called Abbot's Clear. I have modified this glaze with metal oxides. The first has 2 percent cobalt carbonate added. The glaze was poured over, and animated to some extent by the dribbles of thicker glaze.

11 inch (280mm) pitcher, earthenware.Commercial clear glaze with added cobalt, over white slip with chromium oxide.
The pale yellowish pitcher has the same clear glaze as the pitcher above, but has some yellow glaze stain added instead of the cobalt.

9.5 inch (241mm) pitcher, earthenware. Commercial clear glaze with yellow stain over white slip with chromium oxide.
The trouble with the commercial clear glaze is that it is so reliable! It would be ideal if you were producing 100,000 identical press moulded cups an hour on a production line. It is rather like polyurethane varnish, and just stays put in a non-crazed even layer that does not move at all. If the clear glaze is at all thick, it tends to be milky, so it really needs to be applied thinly and evenly. When making an Abbot's Clear glaze I would use 1200 - 1300 ml of water to 1000 grammes of dry glaze material. (By comparison, most glazes I use are between 900 - 1100 ml of water to every 1000 g of dry glaze material).

To be honest, it is a very useful glaze on the inside of things, and can look nice over white slip, but I do find it a bit sad to see it on the outside of an earthenware pot, as I keep thinking how much better a traditional lead glaze would look, very much in the way that lead crystal wine glasses look so much better than the "ordinary" wine glasses do. The trouble is that we are all afraid of lead these days in our grown-up modern society, so.. we have to just accept a boring alternative whilst thinking up other ways to kill ourselves and destroy the planet!

Glaze tests. The one in the middle is the commercial clear glaze with 5 percent manganese dioxide added. The ones each side are both alkaline blue glazes over white slip.
I sometimes make little flower pots 4 or 5 inches high (100 - 125mm), and find them very useful as glaze testers for earthenware glazes, and they are lovely afterwards to use in the garden!

Some Cones that were mentioned in the text.

*Cone 3 is about 1152 C (2106 F) when heated at 60 C/hr (108 F/hr)
**Cone 10 is about 1285 C (2345 F) when heated at 60 C/hr (108 F/hr)
***Cone 6 is about 1201 C (2194 F) when heated at 60 C/hr (108 F/hr)

Insight Glaze Software
Do visit The Digitalfire Corporation website for information about this glaze software.


The Age of Steam!

As a reward to those of you who have bravely struggled through this post that has been mostly about a glaze.... (yawn!)...  I have put some photos of a steam engine that passed by our village last Saturday. The train was a special day excursion trip from Dunedin to Oamaru and back. Such things happen very rarely here, and it is always a pleasure to see a few hundred tons of heavy metal huffing and puffing past with a good head of steam and a cloud of spicy and aromatic coal smoke blessing the air!

Ummm... Sadly the steam engine did prove rather embarrassing, as it started numerous scrub fires beside the track well North of here whilst on its return journey, and ended up having to be towed home behind two diesel powered locomotives.

However.... the brave locomotive still had steam enough on its return journey to wail and moan a splendid ear jangling greeting as it passed us!

For more about the steam train and its fiery day out... here is a link to TV3's news article Oamaru steam train fires under investigation.


June Perry said...

Very nice jug and lovely copper glaze.

Peter said...

Thank you so much for that June, lovely to hear from you.

Arkansas Patti said...

That cobalt glaze really caught my eye. Just lovely.
Goodness, steam engines really put out the smoke. Pretty sure passengers in the cars didn't open the windows. Didn't realize they caused fires. Can see why they are mostly relics today.

Tracey Broome said...

As you may remember, I have always been a fan of turquoise glazes. The pitchers are beauties! I have a similar glaze I use for rake, with frit 3110 and leaving out the feldspar. Very successful rake glaze.

Peter said...

Hi Patti,
Good to hear from you. There was a lot of smoke with that engine, not sure if it was low grade coal that they were using or if it was an early sign of a problem that cropped up later on the trip. Evidently the engine did break down on the home journey, and maybe it was dropping hot embers when it wasn't feeling so well! I think that the other part of the problem is that not enough is done about getting rid of weeds and other combustable matter that is close to the track.

Hi Tracey,
Lovely to hear from you. Yes, I remember your raku work and the turquoise glazes. I keep wishing I could bless these pots further with a little reduction. I may even have a play with refiring some in a small wood fired kiln.

Michèle Hastings said...

I really like the look crazing too. Often times our shino glaze with craze. It doesn't bother me at all to use it for food.
I really like the jug with the slip and clear glaze. They are all beauties, but for some reason that one really appeals to me.

Peter said...

Hi Michèle,
Shino is an example of a glaze that really comes alive when it crazes, pin holes, crawls, and gets a nice flash of flame over it! Laura's favourite mixing bowl is a shino bowl I made some years ago where the glaze crawled a bit and I was not sure at the time if anyone would like it, but it has become really good in the kitchen. Thanks for the mention of the jug with the clear glaze. I have glazed some more jugs with a similar glaze this afternoon and have them in the kiln now just starting their firing.

Melissa Rohrer said...

I tried a commercial clear glaze several years ago. Actually quite a few years ago. I quickly gave up on it because of the cloudiness. I thought it was just the way it was supposed to be. After reading your post maybe I'll give it another try and mix it thinner than usual.
Wonder if fires were common when steam trains were running way back when?

Peter said...

Hi Melissa,
I was really surprised to find just how thin the "clear" commercial glaze had to be before it would fire almost clear. I think that some of my surprise was because most of my experience has been with stoneware temperature glazes that are made with feldspars and the like, and they generally are used much thicker than the frit based ones for earthenware temperatures. I did a lot of tests a couple of years ago of glazes based on frits, and also the commercial glaze that is available in our part of the world, and it was a most frustrating and costly exercise... but it did help in the end! One of the most valuable things I did learn through it all was the importance of finding the correct firing temperature for our earthenware clay to ensure a good glaze fit! This was rather more precise than the wide temperature range that was indicated on the clay bag!
Sad about the fires caused by the steam train. Whilst there was a problem with the engine itself, I think track maintenance is a real issue here now. This used to be done much better in the good old days when rail was State owned, but everything got more shabby after it was privatised.... Clearing weeds doesn't create such an obvious money trail for the shareholders!

Anna said...

lovely colour glazes there.. interesting that your rail is no longer state owned and yet is more shabby.. I wonder what ever happened to the idea of Governments providing a service to the tax payer...

Peter said...

Hi Anna,
Good to hear from you. NZ has had a succession of Governments who seem to have extreme difficulty with the notion of providing a service..., it is a philosophical thing with them... the very thought of being a service provider upsets them and brings them out in a rash! To make sure that there is no risk of them being tempted to do anything other than legislate ... they have systematically sold the Government assets... In fact I am surprised when they still sometimes find another State owned anything to sell off! We live in a land of "user pays", and have (what our PM calls) "a rock star economy" that imports nearly everything, and manufactures very little (apart from milk!), we have become a "knowledge economy"... which has a nice ring to it... (a hollow one!!) Ha, ha...