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Old 09-12-2019, 11:30 AM
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bobc100 bobc100 is offline
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Re: Question about glazing

Quote:
Originally Posted by sidbledsoe
A thicker layer has a higher oil content than a thinner layer of that same paint mix, thus the thicker layer must be considered fatter...
Based on this logic, you should almost never be able to put a very thin layer on top of anything because even a layer with significant oil added would probably contain less total oil content than a significantly thicker layer underneath (remember that this is after you wrote that total oil content and not relative amount of oil is what counts, so the statement quoted would also apply to different paint mixes).

Quote:
Originally Posted by sidbledsoe
I said that I do just that, I did not say anything about a stipulation that required a wait of 6-12 months.
You said you "wait for things to dry very well". This is a vague statement which some might interpret as meaning 6-12 months, since that is the time most recommend for varnishing a painting. You then talk about a painting which you don't have to worry about putting a thin layer on because it's been sitting around for years, thus adding to the impression that a very long time is needed in order to be able to put a thin glaze over a thicker underpainting.
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Old 09-12-2019, 03:48 PM
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Re: Question about glazing

Quote:
Originally Posted by bobc100
Based on this logic, you should almost never be able to put a very thin layer on top of anything because even a layer with significant oil added would probably contain less total oil content than a significantly thicker layer underneath
Dimensional change in any paint layer follows a drying curve that ultimately levels off to a very stable condition.

To significantly reduce the possibility of cracking, you should allow enough time for the lower paint layer to dry or cure, and hence, to undergo the major dimensional flux, you can then paint a thinner, leaner paint layer over top of a thicker, and previously, a fatter paint layer.

Here is a good reference for following the basic layered oil painting rules, this is a quote regarding the thick over thin guideline:
Quote:
When painting with heavy colour, it is best to apply thick layers over thin layers, this is because the thin layers dry quicker. For example if you like the impasto style of the Impressionists with their thick bold brush strokes then it is important to remember that these thick layers need to be upper most – thin layers on top of impasto layers are likely to crack.
Drying time is as important a consideration:
Quote:
It is best to use fast drying colours continuously as under layers. If a fast drying layer is applied on top of a slow drying layer then your painting may crack. This is because the fast drying layers will have dried on top of layers that are still in the process of drying out, and as the slow drying layers dry, they will pull and twist those (fast drying) layers above causing them to crack.
I explained why thicker paint will undergo more dimensional change, over a longer period of time, and thinner paint will have less change, and quicker.
You want the foundation to be more stable than the upper layer(s).
If the lower layer is still under flux, and the upper layer(s) are applied and they cure first, then the lower layer will move and shrink and the upper layer(s) may crack. This is the reason for painting fat over lean, thick over thin, and slower drying over faster drying.
Here is some more good information from Golden on how much film thickness matters, complete with charts.
Film Thickness:
Quote:
In the next graph we show the average percentage of weight gained by multiple samples cast at three different thicknesses.

Quote:
(remember that this is after you wrote that total oil content and not relative amount of oil is what counts, so the statement quoted would also apply to different paint mixes).
Here is what I wrote:
Quote:
It is complicated and you must consider more than a single aspect.

Quote:
You said you "wait for things to dry very well". This is a vague statement which some might interpret as meaning 6-12 months, since that is the time most recommend for varnishing a painting. You then talk about a painting which you don't have to worry about putting a thin layer on because it's been sitting around for years, thus adding to the impression that a very long time is needed in order to be able to put a thin glaze over a thicker underpainting.

You absolutely can't be specific about the time frame, it could be a thin fast drying umber layer with alkyd that dries very fast, or a very thick layer of cad yellow in walnut oil that dries over weeks.
I showed an example and described what I did with this specific painting, I didn't imply that everyone must follow this singular example.
Drying time can be affected by
1- the oil content of the paint
2- the nature of the pigments/oils used
3- the temperature of the environment
4- the mediums used
5- the thickness of the paint layer
So to be more specific about a time frame would be foolish.
The 6-12 month time frame reference is as vague as anything I said,
so which is it 6, 7, 8, 9, 10, 11, or 12 months?
12 month is double or twice the time as 6 months
I may only live another 5 months

Last edited by sidbledsoe : 09-12-2019 at 04:21 PM.
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Old 09-12-2019, 06:15 PM
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Re: Question about glazing

Quote:
Originally Posted by sidbledsoe
Dimensional change...
Let's see if I can simplify.

1. Nearly all sources I've encountered which unambiguously define fat vs. lean define it as pigment to oil ratio and not the total oil contained in a specific layer.

2. Nearly all sources I've encountered treat thick over thin as an issue separate from fat over lean, and they don't apply it to the glazing process.

3. Using these definitions, violating fat over lean in the glazing process occurs if the oil to pigment ratio of the glaze is less than that of the underlying layers. I've never found a single source, other than yourself, which claims that glazing ever violates fat over lean purely because it's a thin layer of paint over a thick one.

I'll admit that you have a very impressive use of vocabulary , but item number 3 is consistent with every other source on glazing I've encountered, so I'm afraid I'm going to have to continue to believe it.

Last edited by bobc100 : 09-12-2019 at 06:29 PM.
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Old 09-13-2019, 01:02 PM
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Re: Question about glazing

Quote:
Originally Posted by bobc100
1. Nearly all sources I've encountered which unambiguously define fat vs. lean define it as pigment to oil ratio and not the total oil contained in a specific layer.

This is the source of the confusion.
Here, you are talking about a different concept than what this thread is actually about.
Your oft quoted, but very ambiguous definition refers to the pigment to oil ratio.
It is specifically, and only concerned with fat.
Specifically, what is a fat versus a lean paint?
That definition of a specific paint mix does not define the sound fat over lean principles that one should follow in the process of layered painting.
The poster and I, in this thread are not referring to the oil pigment ratios, we are talking about the process of layered painting, and doing that layered painting in a manner that follows the principles of fat over lean layering, or, the Fat Over Lean Rule:
Quote:
Originally Posted by MarcF
(trying to observe Fat over Lean principles)
Quote:
Originally Posted by sidbledsoe
you are concerned with painting thicker over thin, to follow fat over lean.

The poster obviously understands that painting thicker over thin, does follow that aspect of the basic principles.
You will find many reliable sources that understand the mass confusion that this ambiguous definition has given rise to, regarding the process of layered painting. What if you use is an alkyd? what if you use stand oil vs using poppy oil? what if you use a faster drying oil vs a slower drying oil? What if you use a solvent, because it evaporates and leaves the oil/pigment ratio the same as it was? What if you paint a thin layer over a thick layer?
What if you paint over a layer that isn't dry yet?
this forum is absolutely rife with such questions, and many more. The threads are far too numerous to list, one example:
http://www.wetcanvas.com/forums/showthread.php?t=272220

In an effort to do away with this narrow, all too specific, very flawed, and antiquated definition, reliable sources are trying to get away from it and
go with a better definition of the process itself.
Such as this, found here:
Quote:
Fat-Over-Lean Rule, oil:
Always paint flexible “fat” layers over less flexible “lean” layers.
and from Winsor Newton:
Quote:
Fat Over Lean
Each successive layer needs to be more flexible than the one underneath. Think of the rule as ‘Flexible over Non-Flexible.
From Natural Pigments, on the Fat over Lean Rule:
Quote:
This rule appears to confuse so many artists or ignored completely by others. Perhaps a better way to express the rule "always paint fat on lean" is always paint a slower drying paint film over a faster drying film. Think in terms of the last applied paint film being more flexible than the paint film underneath.

There they are, reliable sources that define the Fat Over Lean Rule, not the oil/pigment fat ratios of paints.
You can clearly see that the correct way to paint fat over lean layers is to consider all the factors that matter, and to only consider the paint mix fat content just doesn't do that.

Quote:
2. Nearly all sources I've encountered treat thick over thin as an issue separate from fat over lean, and they don't apply it to the glazing process.
Then you should seek out more sources, for it is not a separate issue, it is the very same issue, painting more flexible over less flexible includes thicker and slower drying paint layers.
Quote:
3. Using these definitions, violating fat over lean in the glazing process occurs if the oil to pigment ratio of the glaze is less than that of the underlying layers. I've never found a single source, other than yourself, which claims that glazing ever violates fat over lean purely because it's a thin layer of paint over a thick one.
I encourage you to continue looking. It is one of the most oft contradictions found regarding layered painting.
Others here and MarcF have done it in a number of previous threads.
http://www.wetcanvas.com/forums/showthread.php?t=59635
http://www.wetcanvas.com/forums/showthread.php?t=558391
http://www.wetcanvas.com/forums/show....php?t=1092552

Quote:
item number 3 is consistent with every other source on glazing I've encountered, so I'm afraid I'm going to have to continue to believe it.
Recall that even with beliefs so firmly entrenched, you repeated the quote by the OP in post #4 :
Quote:
3rd layer even thicker (trying to observe Fat over Lean principles)
yet with your response to that comment, there was no questioning, comment, nor attempt to correct, but have fervently assailed my simple reply to the poster.
I think that in reality, like the poster understands, you also do intuitively know that a thicker layer does indeed mean a slower drying, more oil (fat) containing layer. If there is more oil in a layer, then there is more fat, there is no possible way to get around this fact.

Last edited by sidbledsoe : 09-13-2019 at 01:17 PM.
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