MITRA Forum Question Details

i am interest in this topic, any expert may help to input?

​ i also looked that youtube section and believe the host is George, maybe he can chat in here to explain out any confusion when people not agree with what said? ​

Yeah, i hope George can comment here as well. My question would be most time we are painting with color that dries faster on top of color that dries average to slow speed. 

for instance, like in underpainting if you use titanium white, which is an average to slow dry color, then probabaly will mix some earth colors which dries very fast on top, then wont you really see a problem if you paint straight from tube in this way? so i want to ask you guys what you will do in this case?

Maybe:

1, you purchase similar colors in fast, medium and slow dry color catagory and place them in order?

Or, 2, you add oil to faster dry color to make it dries slower if you want to place it over average-slow dry color?

Or, 3, you completely not care drying speed and use whatever color needed in any order?

​hope to get some insights in the community. thanks. 

Hello there,

Fat over lean is easily simplified as slower drying over faster drying. It's more about making sure you have over time more flexible over less flexible. The main parameters are thus logically the drying rate of the colour and the amount of oil used AND the time you allow for drying between each layer.

By reasoning backward, if you want your paint to crack, use a slow drying colour with lots of oils, and when it's barely touch dry, layer it with a lean and fast drying colour.

Back in proper order that means the longer you wait for each layer to dry, the lower the chances you take to break the rule unknowingly.

The main idea then in my humble opinion wouldn't be to just paint with colours straight from the tube, but rather to gradually shift your "fat" content from layer to layer you paint to increase the likely flexibility of the overall cake film.

Meaning thin your painting mediums with spirits first, and gradually shift your spirit to oil ratio toward more oil on each layer you go. The longer you wait between each layer the better.

One other detail, if art suppliers don't generally mention the drying time of their oils, if you ask their support team, they should give them to you. At least what the average time they'd expect. 

PVC or oil content is a thing but art suppliers add driers as well. between that and pigments drying the films differently and with varying flexibility makes it all the more harder to accutely define everything.

Mixing your colours with painting mediums eases out and softens the differences. Waiting patiently for each layer to dry reduces the risks as well. And finally painting thinly also helps reducing the risks.

If you need to put a fast dry colour over a slow drying one: give the time for the slow one to dry, and give plenty of oil to your fast drying one. Simply.

Cheers and good patience to you,

Lussh

  hi Lussh, thanks for your answer, i have few questions:

1, how dry would be the safe dry? i remember George from Natrual pigment says "hard dry" but i have no place to find what means by hard dry. and how to differentiate from touch dry? to me, what i can see is the phase that no longer pass through paint to finger when touched, will this be considered "hard dry"?

​

2, according to Michael Harding, some color is both very fat and very fast dry, so is maybe easier to thin it a lot for initial layer or add lot of oil to slow it down for final layer, but how about use in middle layer? or should we just avoid using this kind of complex color in middle layer?

Hello there,

The safeest dry like for varnishing would be one year give or take. Nobody is that patient obviously. As much as the subsequent layers allow for some air to permeates through and allow the drying, it becomes slower or even impaired depending on what you put on top (varnishes or resins for instance).

Hard dry means "reaching the plateau of mechanical properties (hardness and flexiblity or lack of) over time". 

Oil painting as it dries first becomes touch dry "rapidly", and then grows over time into its reticulated film until reaching a plateau of properties (or gradually falling back as it ages even). Hard dry means the polymer is formed and mostly won't react anymore.

How to differentiate from touch dry, it's the tip often given before varnishing: wet a cloth with some solvent and wipe your colour. if it's still very solvent soluble and comes off easily, it's not fully hard dry yet. 

The problem you raise is, this is not a binary property like wet switching to fully dry. It's a gradual progress that varies from pigment to pigment, from oil to oil, from drying condition, amount of resin mixed in painting mediums, thickness of layers and time between, and so on. So many parametres that all you can do is chose how much patience you're willing to give and the wet cloth will always be able to remove some oil colour regardless.

The more parametres you control the higher your chances, but you'll never be 100% sure.

However it's still a logarithmic increase of properties, (or exponential decrease), meaning between waiting one day and one week the change is huge, but between one month and one year, the properties change is marginal. So don't beat yourself up for just waiting "just" a few weeks between layers, it's plenty good already. Aside the amount of colour that solvent can pick up, you'll never really know for sure.

​Very fat and very fast drying is possible, although I don't have a specific colour coming to mind.

From what you're saying, yes, thin it for the first layers, fatten it for the last, and go for ratios in between for the middle layers and you'll be fine.

If you also give plenty of time for your layers to dry before adding another (reaching enough of a hard dry), it's less of a concern.

Cheers and patience,

Lussh

Hi Lussh​,

thanks for your reply. according to Michael Harding, the Phathalo group color is very fast drying as well as very fat, so seems to me it is very hard to use as it lays property at 2 ends. but using other blue is fine to me, because it is overwelming color to me anyway. 

but for PR 101 Venetian Red or transparent oxides are kind of very useful color to me. 

According to MH, Venetian Red is lean but dries fast, so probably mix it with little oil to slow it for color layering like fresh tone. 

but for transparent oxide, which are fat due to small lake alike particle size but dries fast, so it is another example of hard to use color:

​because the transparency is great for glazing, then if used on top layer, its drying speed is fastest, then as you said, had to wait like month to do this layering.

but if want to use it in middle layer, i feel its is hard to fine tune it, because if you want to slow its speed, you add more oil to it, but since itself is pretty fat, then adding oil will make it very fat, but it is still in middle layer, so means the following layer will be even more fat, i am afraid then it will beeding up very quickly.

Hi there,

Phthalo pigments aren't fast drying in the way I would have put it but I see the intended meaning. 

They're not natural driers and they don't impair the drying much either to the best of my knowledge.

but because they are particularly powerful, your oil colour with phthalo pigments only has a few % of pigment (in weight). They're among the colours with the less pigment overall (because super strong colorant).

So it's more that their colours have mechanically more oil (and not too much filler to avoid staining the transparency). 

So they're very fat in that they have a lot of oil mechanically against other colours, and they're fast drying because... they have more driers added.

The difference however against fast drying colours that are lean like iron oxides, is the flexibility of the dry film.The leaner the colour, the less flexible its thick film will be. Phthalos are good since they will give you a flexible film. Iron oxides will need the care of more oil if you layer them above. But it's about their flexiblity more than their drying time.

If you allow each layer to dry fully, your concern is the flexibility of the film to go crescendo. Your phthalos won't be an issue no matter where you put them (so long they have time to dry through). 

Your lower layers of lean colours neither. Your mid or top layers of lean colours might. Give them som oil yes, "some" for mid layers, "a little more than some" for the top layers.

I hope this helps, 

Cheers,

Lussh

Hi Lussh, you make another good point regards film flexibility, these info never released from supplier as far as i know, so if you may share common materials in terms film flexibility?

​also, i am not sure why people saying flexible on top, inflexible on bottom (suppose each layer dry enough in between), i mean the canvas can bend inward and outward, which may require more flexible layer on top or more flexible layer at bottom depend on bending direction?