Monday, 27 August 2012

Post 33 Glaze application

You'd have to agree that a bonsai pot that's 350 to 400mm long is not something that you would ever think about glaze 'dipping'. Just imagine the volume of glaze you'd need, let alone begin to think about keeping the glaze off the base and inside surfaces.

The only two practical options are spraying and brushing.

I like the idea of spraying to get an even smooth surface but you do need a ventilated booth to do this best. There is a lot of glaze wasted too, which I don't like, and then more importantly how do you know when you'ge got enough glaze on the pot?

So it's brushing for me.
That still leaves the probelm of how much to put on and how to get a smooth application.
I read up on glaze SG to get the right composition for brushing etc etc but HOW MUCH DO YOU PUT ON #@$@$??? Obviously you work it out by trial and error until experience becomes intuition and you just know. Well that's all very well but as with most things I'll be convinced when you "show me the numbers". Getting the data sorted also offes a level of repeatability. There's nothing quite like having a reliable rule of thumb to apply.

I know from my test tiles how many 'coats' to put on and when I do the same on the pots I have begun to know how much I have applied for that number of coats. For the pots I've glazed recently I've nearly applied around 100gms of equivalent dry glaze material. That is I mixed up 100 gms of dry components to make a batch and nearly applied it all to the pot. Taking some measurements on the pots to work out the surface area over which the glaze has been applied reveals an application rate of about 600 to 650 gms of dry glaze to each square meter of surface. I haven't been able to find any similar advice or benchmark in anything I've read anywhere on the internet. So work out the area to be glazed, multiply by 600 to 650, make a batch with that much dry materials and then put it all on the pot. Problem 1 solved. Some glazes do need a heavier application than others and so I'm afraid trial and error and careful observation will tell you which is which.

If anyone has their own rules of thumb about glaze application I'd love to hear from you and will publish the results. There are a lot of folks who are interested in and looking for reliable glaze recipes. That will take you a long way but you still have to get the application right too.

If brushing is the answer then Problem 2 is about which brush to use.

All the advice available says to use a really soft brush, fill it up with glaze and slap it on. The topmost brush in the photo is the usual sort of brush available for the purpose in the pottery supplies shops. Yes the bisque does suck the glaze onto the surface, particularly for the first coat, but what a heavily laden soft brush does, because it can't actually move the glaze over the surface, is deposit a very uneven glaze depth, which on firing translates to a bump in the surface each time you hit the pot with a new brushfull. Very unattractive.

My next trial was a high quality sable artist brush called a mop. Slightly more controllabe and nice to use but generally the same result.
This took me right back to when I started and was just 'having a go' using a regular domestic paint brush for acrilic paint - a synthetic one with standard firm bristles. Because it is firm you can spread the glaze and get a smooth application at all the junctions between wet and dry, glazed and not.
If you are just trying to work on the basis of a number of coats, then with a firm brush how many do you apply?
Well the answer is that if you know how much glaze you need to put on the pot (probelm 1 solved above) the number of coats is academic - just keep putting on thin layers and get the best surface finish!

Finally I have found that a final burnish with a finger or soft piece of cotton fabric will remove any residual brush marks.

So my new rule of thumb is 600 to 650 gms/square meter and a firm brush to apply the full glaze batch is the summary of my limited but tested experience -  for now! What's yours?

Tuesday, 21 August 2012

Post 32 More glaze tests and recipes

This post covers a few additional recipes that produced good satin / matte surfaces, from my recent fourth test series.

The first two sets are derived from John Post's "Pete's Tomato Red" glaze, adapted for our frit substitute. One uses Spodumene and the other not.
If you want to use these reipes where Frit 4108 is not available the original recipe calls for Frit 3134, but you'll need to go back to the source for the balance of the conmponents. I've moved things around a little to try to get the unity formula equivalence in the recipe.

So the first set without Spodumene, unity ratio 6.2.

Nephylene Syenite                  22.6
Whiting                                      7.5
Silica                                         20.7
Frit 4108                                   10.6
Kaolin                                        20.4
Bentonite                                    2
Bone Ash                                   9.8
Mangnesium Carbonate          6.4

4-78 with Copper Carb 0.5%, Cobalt Carb 1.5%, Rutile 6%
4-79 with Copper Carb 2.5%, Rutile 6%
4-80 with RIO 0.5%, Cobalt Carb 0.5%, Nickel Oxide 4%
4-81 with RIO 3.5%, Rutile 6%
4-82 with Titanium Oxide 3%, Tin Oxide 2%, Manganese Carb 3%

 The next set are the ones with the Spodumene substitution, unity ratio also 6.2.
Nephylene Syenite                   12
Whiting                                       8
Silica                                         20.5
Frit 4108                                   12.3
Kaolin                                        21.6
Spodumene                              11.2
Bone Ash                                    8
Mangnesium Carbonate          6.4

4-83 with Copper Carb 1%, Titanium Oxide 4%, Manganese Carb 4%
4-84 with RIO 4%, Rutile 6%
4-85 with Copper Carb 4%, Rutile 6%
4-86 with Copper Carb 4%, Cobalt Carb 1.5%, Rutile 6%
4-87 with Nickel Oxide 4%, Zinc Oxide 10%

These ones had a little more glossy surface.

The last set are derived from a BTM glaze which I have modified to go satin. Unity ratio 6.6.

Nephylene Syenite                   40
Silica                                         10
Ball Clay                                    15
Talc                                             14
Kaolin                                          6
Wollastonite                              15

4-88 with Copper Carb 0.75%, Cobalt Carb 0.75%, Rutile 6%
4-89 with RIO 1%, Copper Carb 2%, Nickel Oxide 1.5%, Rutile 6%
4-90 with RIO 11%, Zinc Oxide 8% (you can see the zinc crystallisation in the heavier glaze application)
4-91 with RIO 1%, Copper Carb 2%,Nickel Oxide 2%, Tin Oxide 6%

Of these both 4-88 and 4-89 have given a very useful result, a great combination of colour, opacity and surface.

My next challenge is to choose one of these glazes and then use it to do a series which tests a wider colour response. Plans are in hand for about 60 test tiles.

Wednesday, 15 August 2012

Post 31 Bonsai root pruning

On the 1st of September spring arrives here with a rush every year. It's like a starting signal for rising temperatures and very quickly the temperature can get to the late 20s (C). It's not unusual for some deciduous species to start shooting new leaves in August. Some have even held their leaves over winter. For my deciduous trees that need root pruning I like to do them now. Last year work got totally in the way and so by this year most of the trees have been in the same pots undisturbed for 3 years, well overdue for a trim under the water line. I've got some work ahead of me and the pot making is relegated to the back burner for now.

I thought a little series of shots of the process might be fun.

This tree is a Celtis Sinensis or Chinese Hackberry. Looked down on by some but they are eminently trainable and make beautiful bonsai. It's over 25 year old.

To start here it is before root pruning. At this point I've pruned the branches, The wires on the lower left branch are there to remind me where I want to develop a little more branch mass as it fills out with new foliage.

The pot is a pretty chunky glazed chinese pot. Not a bad choice but the tree would be better suited to a more shallow pot - one of mine! The tree has a well developed nebari, and good taper the result of yearly root pruning in its youth and gradual development of height.

I start with root pruning tool number 1, the machete.  If you are confident about having a good root mass of fine roots then this is a good quick way to halve the root mass. Not to be used with a young tree or one where there is the chance of a few dominant long roots.
First off comes the base.

Then around all four sides. The base and edges tend to accumulate a greater root mass so this gets it off fast.

Then a good water jetting to remove the soil. Looking at the photo above you might think there wasn't much root activity left in the soil still attached to the plant, but give it a wash and you can see just how much there is - a tightly woven mass.

And from underneath.

Next comes the further thinning, firstly getting right back to the underside of the trunk and then creating a good radial disc of roots to service the tree and further develop the nebari and taper.

Here you can see the underside of the trunk and how the roots principally come from the outer edge of the nebari. For the uninitiated this looks like certain death, but in reality the tree manages well and in a few weeks will be in full leaf. The process is not much different from what is done with bare root sold roses or fruit trees. The advantage here is that over previous years primary roots have been eliminated in favour of a good distribution of minor and feeder roots close to the trunk.

And finally re-established in it's pot. The pot is one of mine - number 19 from Post 27. Not exactly a studio photo but you get the impression. Both the tree and the pot look so much better together.

Here's another one to show again the radial disc fine root development for a fast recovery from pruning and ongoing nebari development.

Friday, 10 August 2012

Post 30 Glaze testing & recipes

Here are some of the results of my recent glaze trials; all cone 6 to 1240C.

All the glaze formulations in this post have produced a buttery soft satin matte finish with the same look and feel over the whole surface. All of them offer exactly what I'm after for pot glazes in application and finish. The colours are all muted, almost pastel variations which again are perfect for bonsai pots, not wanting to outdo the appeal of the tree!. My next series of trials will concentrate on these few base formulations with a wider gradation of colourant variations.

The first three shots are of variants of John Post's high calcium matte cone 6 glaze.

I've modified the original formula to accommodate the available local Frit and also to give the glaze that touch of satin finish I've been chasing. I also formulated a number of variants to test the alternatives.

The first set use the following base:
Nepheline Syenite        31.5
Whiting                           4.9
Silica                              12
Ball Clay                         22.6
Frit 4124                          5
Kaoliin                              8
Wollastonite                   16
The unity ratio is 5.82.

 Additions to the base glaze are:

4-1  Copper Carb 0.5%, Cobalt Carb  0.75%, Titanium Dioxide 2%, Rutile 6%
4-2  Cobalt Carb  0.5%,  Titanium Dioxide 4%, Manganese Carb  5%
4-3  Red Iron Oxide 4%, Titanium Dioxide 2%,  Manganese Carb  4%,  Rutile 6%
4-4  Copper Carb  2.25%,  Cobalt Carb  0.25%,  Titanium Dioxide 2%, Rutile 6%
4-5  Titanium Dioxide 2%,  Manganese Carb  4%, Rutile 6%

The next two have a base glaze composition of:
Nepheline Syenite        20
Whiting                           7
Silica                              16
Frit 4124                         5
Talc                                  5
Kaoliin                            29
Wollastonite                   18
The unity ratio is 5.93.

 Additions to the base glaze are:

4-8  Cobalt Carb  0.5%, Nickel Oxide 1.5%,  Titanium Dioxide 2%, Rutile 6% (a very attrqctive pastel green with a little Rutile speckle)
4-9  Red Iron Oxide  4%, Tin Oxide 5% (good for breaking variegation on surface relief)

The next two have a base glaze composition of:

Nepheline Syenite        14
Whiting                           7
Silica                              15
Frit 4124                         10
Talc                                  4
Kaoliin                            32
Wollastonite                   18
The unity ratio is 5.82.

4-10  Nickel Oxide 1.5%,  Titanium Dioxide 4%, Rutile 6%
4-11  Red Iron Oxide  4%, Tin Oxide 5%

The next two sets derive from a Digitalfire formula, (G2571A).
The next set, 4-14 to 4-18, use the following base:

Custer Potash              26
Whiting                          10
Silica                             15
Kaoliin                           24
Dolomite                       15
Wollastonite                   6
Gerstley Borate             4

The unity ratio is 5.9.

 Additions to the base glaze are:

4-14  Cobalt Carb  0.5%, Nickel Oxide 1.5%,Titanium Dioxide 4%
4-15  Red Iron Oxide 3%, Copper Carb 0.75%,Cobalt Carb 1%, Rutile 6%
4-16  Red Iron Oxide 0.5%, Copper Carb 1%,Cobalt Carb 1%,Titanium Dioxide 4%
4-17  Nickel Oxide 2%, Tin Oxide 3%, Rutile 6% (nice creamy tones when thicker)
4-18  Copper Carb 3%, Nickel Oxide 1%, Rutile 6%

And the final set base formula is;

Custer Potash              25
Whiting                          3
Silica                             20
Kaoliin                           25
Dolomite                       17
Wollastonite                   6
Gerstley Borate             4

The unity ratio is 5.9.

Additions to the base glaze are:

4-23  Copper Carb  4%, Rutile 6% (this was one of only a few where the Rutile provided speckling as well as just being an opacifier like Titanium)
4-24  Red Iron Oxide 6%, Rutile 6% (better colour when not too thick)
4-25  Red Iron Oxide 4%,Titanium Dioxide 4%, Manganese Carb 3%
4-26  Cobalt Carb 1%, Rutile 6%
4-27  RIO 0.25%, Cobalt Carb 1%, Tin Oxide 4%

Thursday, 9 August 2012

Post 29 Misty Grey Pot

This is a pot I fired a little while ago but they always look different when they've been pressed into service so it's worth another viewing.

The tree is a ficus, called as I recall 'shorty'. It's not exactly in conformance with classic design principles but an atractive tree none the less. The grey of the glaze nicely complements the trunk / bark colouration. Here is a more detailed shot.

The glaze is a manipulation of Val Cushing's cone 6 base but was far too complex in its formulation to repeat here and besides I wouldn't use it again now that I have trialled  alternatives that I prefer. Interesting however is the additives combined to give the grey colour. It has 2.5% Copper Carb, 4% Titanium Dioxide and 3% Lithium Carbonate. I will use that group again some time in a base more to my liking.

Saturday, 4 August 2012

Post 28 Pyro-plasticity of clay

The clay I use is a Clayworks stoneware (RGH) which has a little iron in it to give it a buff colour when fired. It's a nice fine clay that's easy to use. I have experiemented with a number of lower fired clays - terracotta types that are very 'sticky'; to hands and everything they touch, and RGH is nothing like that.

Firing at the higher stoneware temperatures produces a less porus ceramic material which is good but the higher temperatures introduces the firing risk of pyro-plasticity. Both stoneware and porcelainous clays can have the same characteristic. As the clay approaches the final vitrification temperature the materials of composition form a homogeneous glass like material, and melt just enough to exclude the spaces which ultimately makes the body less porus. This also results in shrinkage.

The photo below is the result of a test to demonstrate the level of pyro-plasticity of RGH. I made a flat strip of clay about 200 x 30 x 12, dried and bisqued it. After bisque firing it was nice and flat. I then fired it to 1230C in a glaze firing ( it is unglazed) supported at each end by two little props, also of RGH.

As you can see it has slumped as it glassified, until it rested on the shelf. It didn't however get so soft or molten to bond to the props.

There in lies the challenge in higher fired clay forms. Gravity is the threat to stability and forms exposed to deformation under its relentless drag need to be supported. Compounding this impact is the contraction that is occuring, making the object slide over the shelf as it shrinks.

In the commercial ceramics industry the formulation of the clay body is the subject of research to find and formulate one which is less plastic and able to withstand industrial firing process without deformation. Here's an interesting paper to show that it is possible.