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Old 07-19-2008, 11:07 AM
CareyG CareyG is offline
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Why do some pigments "dry" faster than others?

Here's something I've never seen adequately explained...why some pigments "dry" faster than others.

Anyone know if this is related to the oil content or something else? (I realize that different pigments will need different amounts of oil to create a good paint consistency.) This can't be the whole story as some paints are oil rich and yet dry incredibly fast...

Just a wondering thought that popped into my head stemming from another thread.

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Last edited by CareyG : 07-19-2008 at 11:40 AM.
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Old 07-19-2008, 11:31 AM
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Itís a good question Carey, Iím still fuzzy on some of the details about that myself.

There are just so many pigments that dry differently for different reason. Some pigments require a small amount of oil but will still dry slow while others will require a much larger volume of oil and dry very quickly.

Why pigments require various amounts of oil to form a paint I donít really know. Part of it has to do with particle size, I believe the more coarse the pigment is the more oil is required to make paint, but I also know thatís not always true. Interesting question though, it kind of a mystery to me. I donít really understand how at times a bit pile of dust requires a tiny amount of oil to make paint or how a small pile of dust may require a much large dose of oil. I am guessing a big part of it has to do with the particles actually absorbing oil like a sponge, where the more heavy metal pigments absorb less oil which is why heavy metal pigments tend to require less oils to make a paint. At least that would be consistant with twhat Ive been seeing.

Some pigments like raw umber or Sienna have and oxidizing element in them that just speeds up that chemical reaction up. Some might have more of that catalyst than others.

Last edited by CareyG : 07-19-2008 at 11:41 AM. Reason: moved topic into a new thread
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Old 07-19-2008, 01:24 PM
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Re: Why do some pigments "dry" faster than others?

My Vasari Sap Green takes forever and a day to dry to the touch...

I've got my listening ears on for this topic... thanks for bringing it up Carey.
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Old 07-19-2008, 01:39 PM
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Re: Why do some pigments "dry" faster than others?

Good question, I have wondered too. I found this:


Watching oil paint dry: How long it takesby Richard Gibson
(remember that oil dries by a chemical reaction (oxidation) not by evaporation like watercolour)depending on what ground the paint is on how thick it is and the atmospheric conditions:
* on canvas in warm dry conditions thin oils can be touch dry overnight
on a nonporous surface (including dry paint)it may take several days
* thick impasto oils will take up to six months.
* six months is always recommended before varnishing.
You can buy special media and oils which dry more quickly e.g. bleached linseed oil thickened linseed oil and drying linseed oil speed up the drying process and
synthetic alkyd resin media can dry very quickly even in impasto.
A further factor is the pigment in the paint itself; the chemical pigment can act either as an accelerant or a retardant of the oxidation. therefore, some colours dry faster than others.
For further information on many aspects of oils see:
"The oil colour book" Edited by
David Pyle and
Emma Pearce,
Winsor & Newton

MORE HERE: http://www.helium.com/items/14623-wa...-long-it-takes
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Old 07-20-2008, 04:32 PM
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Re: Why do some pigments "dry" faster than others?

The subject is fairly complex, or rather, prone to variables.
Primarily, the question is one of oxidation and oil content. The more oil present, the longer paints take to dry. Paints that take a long time are transparent colors, reds, yellows, and blacks. Also, titanium white can take a long time.
Pigments that dry quickly are usually earth colors and iron oxides, particularly the umbers which have manganese as a natural siccative (drying agent).
Mars Black is faster drying than Ivory Black, which in turn is faster than Lamp or Carbon Black. In the case of Mars, it is a chemical advantage of iron oxide. The other two are largely oil content affecting drying time.
Cobalt pigment is also a natural drier, so mixtures with cobalt blue will dry more quickly than most other blues.
A common misconception is that lead pigment accelerates drying times. This is incorrect unless driers have been added.
Temperature and air flow also play a part in speeding drying, with temperature being the superior force. Paints can dry in less than half the time if "dried" at 75 degrees F, compared to 55 degrees F. Roughly, "normal" paint may take up to two weeks to dry at 40-50F, and 1-2 days at 70-90F (overnight low/day high).
The quote above, "thick impasto oils will take up to six months," on drying times in misleading. It doesn't take six months for impasto oils to dry -- they are surface dry in slightly longer times than thinly painted areas. But the interior, far removed from oxygen in air, can take several years to completely dry. But that is really academic to the use of varnish and further paint layers, which can be pursued in the normal time frame.
Average or normal drying times for "average" thickness of layers will be 2 to 4 days. Thin passages can dry overnight. Addition of umbers can dry paints in less than that time.
The use of siccatives is recommended for fast drying of all oil binders, including poppy and safflower oils. Common driers are lead napthanate, cobalt, manganese, and mixtures of these chemical driers. Also alkyd oils dry more quickly than regular oils, but will accelerate regular oils when mixed in. Same for quick drying alkyd mediums.
With the use of siccatives or quick drying mediums some oil paint layers can be safely overpainted in as little as six hours without disturbing the underlayers (gentle brushing, of course!).
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Old 07-20-2008, 04:48 PM
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Re: Why do some pigments "dry" faster than others?

Quote:
Originally Posted by Smokin
Some pigments like raw umber or Sienna have and oxidizing element in them that just speeds up that chemical reaction up. ...
This may be an important factor causing these color oil paints to dry faster.

I think most oil paints are linseed oil based. But some, like Titanium white, uses poppy seed oil, which dry slower.
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Old 07-20-2008, 06:45 PM
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Re: Why do some pigments "dry" faster than others?

Quote:
A common misconception is that lead pigment accelerates drying times. This is incorrect unless driers have been added.

Just to elaborate, Iím sure Gun know this, but lead IS a siccative in many forms like lead oxide, massicot, litharge ect. The "misconception" is assuming that lead carbonate aka (flake white or cremz white) is an oxidizer or powerful siccative. Cooking lead carbonate does convert it into a oxidizer (aka siccative) or more specifically pbo as in recipes for black oil.

Another factor on drying is surprisingly (at least to me) light. Oil paint drying in the dark will take longer to dry than paint exposed to light, ambient or direct.

Below is a table of drying times I pulled from Robert Massey's book. (# = parts Pigment - 1 part oil by volume)

Some of these pigments drying times are easy to explain like for the umbers or Burnt sienna. These pigments have natural oxidizers or siccative that speed up the oxidation process.

The lead, titanium, or zinc whites dry moderately fast imo because of how lean they. There is very little oil there to dry.

Alizarins, Ivory black, and phyalo greens are slow driers imo because of how fat these pigments are. These pigments require a lot of oil to form a paint and are also the pigments that tend to crack as well if not careful.

The mystery to me is why a cadmium, vermillion, or ylw ochre are such slow driers. They arenít particularly fat or lean pigments, yet the drying time is significantly different than other paints I would have assumed comparable. At this time I assume itís either that these pigments have a natural anti oxidant present, or since light is an ingredient in drying time, it may just be how light is absorbed with these pigments that makes the difference.


Quote:

My Vasari Sap Green takes forever and a day to dry to the touch...

Kinda hard to comment. Iíve heard from gun that they have a chart that list their pigments they use, but Iíve never seen one. I assume that this sap green is made of ultramarine and a synthetic Indian ylw. Both pigments known to be slow driers. Why exactly its slow drying I dunno.
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Old 07-21-2008, 02:07 AM
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Re: Why do some pigments "dry" faster than others?

I was only addressing lead-based paint, not variations that can be used as siccatives.
I don't know that I fully agree with this Massey list. I've never noticed Yellow Ochre being particularly slow drying. Same for Cerulean, which is cobalt-tin pigment.

Merlion -- I'd say most titanium is ground in linseed or safflower. Poppy would be very limited today, with Blockx currently the main supplier of poppy-based whites.
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Old 07-21-2008, 09:42 AM
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Re: Why do some pigments "dry" faster than others?

Both litharge and massicot have a long history of being used as a pigment/paints. Its only in modern times did they lose favor. Bit of trivia, but in the mid west if you can imagine that typical farm with a rust red silos or barn. Thatís actually red lead, aka litharge or pbo4.

As for the list, I donít know how accurate it is either. Iíve never spent any time tracking drying times of all my paints & doubt I ever will. Can say though after mixing paints and observing what an unmodified oil paint acts like, the efforts made by most paint manufactures to get paints to act similar to each other and dull a lot of their specific characteristics is a lot more obvious now. With unmodified or less processed paints, fast drying pigments seem to dry fast, slower drying pigments seem to dry slower and all those individual characteristics stand out.
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Old 07-21-2008, 10:24 AM
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Re: Why do some pigments "dry" faster than others?

Certain metals like cobalt, manganese, iron or lead salts act as catalysts to the oxidation of oil causing it to cure more rapidly. That's why they're often sold specifically as dryers as well as pigments. Also, what designates an oil as drying or non-drying is it's iodine content. The fatty content of the oil is also an issue with how fast it dries, and that may vary even within a particular type, like linseed, depending on how it's processed.

...or so I've read.
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