THE ACRYLIC DRYING
Water is Forced Out by Capillary
Acrylics dry as the vehicle
that carries them, mostly composed of water, leaves the film.
As water evaporates or is absorbed by the substrate, tiny acrylic
polymer spheres are forced into ever closer contact. Eventually
they are crowded so tightly that the spaces between them create
capillary forces, and water is pulled from the paint film. This
capillary action packs the acrylic spheres against one another
in a honeycomb-like pattern, and they begin to form a continuous,
cohesive film. As this occurs, the polymer spheres, composed of
long chains of acrylic, actually deform and partially combine
with one another in a process of film formation called coalescence.
The Two Drying Stages of Acrylics
The drying of acrylic
paints occurs in two very different stages, hence drying times
must be thought of in two different time frames. The first stage,
a relatively short period of time, results in the formation of
a skin over the surface of the paint. This is the time that it
takes for acrylics to "dry to the touch". At this point,
the flow of water towards the surface is no longer sufficient
to keep the paint film wet. Very thin films can feel dry within
seconds, while thick films may take a full day or more to skin
The second stage of drying is the
time for the entire thickness of the film to be thoroughly dry.
That is, the time required for all of the water and solvent (used
as freeze-thaw stabilizer and coalescent) to evaporate and leave
the film. This is a most crucial time frame, as the ultimate physical
properties, such as adhesion, hardness and clarity, do not fully
develop until the film is near complete dryness. For very thin
films, this time may be a few days, while films of 1/4 inch thickness
or more will take months and even years to be completely dry.
Many artists are not aware of this
more lengthy drying time. This is the reason that one may find
that a rather thick layer of paint has not adhered to the surface
when tested a day or two after application. This same layer of
paint will also seem very soft. The skin may have dried sufficiently,
but the paint in the center is still wet. Regarding development
of clarity in gels and/or mediums, one can allow a painting to
clear, store it away and later notice that it has become cloudy.
The film may have only been partly cured, and is soft enough to
allow moisture from the air to penetrate, turning it slightly
milky again. Given enough time for more complete drying, these
properties should improve dramatically.
INFLUENTIAL DRYING FACTORS
Temperature, humidity and airflow
are environmental factors that influence the drying times of acrylic
paint films. By controlling all of these factors, the artist can
make use of their advantages.
Ideally, the temperature
should be around 70 to 90oF during the drying/curing process.
Temperatures below 49oF (9oC) will not allow the polymer solids
to properly coalesce to form a continuous film, and may result
in film failure (cracking, adhesion failure, powdered film, etc.).
Higher temperatures, like those reached with a hair drier or heat
lamp, can speed drying times up significantly, but overheating
can cause bubbling or burn the acrylic film. Likewise, lower temperatures
will slow down the drying process and can be used to one's advantage
for increasing the working time of the acrylic paints.
Relative humidity in
excess of 75% will slow the evaporation of water from the surface,
slowing down the drying process. Temperatures of 70 to 85oF and
humidity under 75% are ideal for drying.
While a moderate and
steady airflow in the drying area is favorable for thorough drying,
a strong breeze, especially one directly on the paint surface,
can cause film formation failure, such as cracking and/or wrinkling.
Utilizing the Quick-Drying Tendencies
The naturally quick drying
time of acrylics can also be a great benefit. Many of the most
successful uses of acrylic paint come from people taking advantage
of this property, which allows for painting over almost immediately.
Unique acrylic glazing techniques, hard-edged applications and
quick manipulation of the painting surface would not be possible
without these very rapid drying properties.
Application of Additional Layers
of Acrylic Paint
There are no limitations
on applying additional acrylic paint layers. That is, an artist
can apply acrylic paints while the underpainting is still wet,
just skinned over, or has dried for several years. If one is going
to apply oils over an acrylic underpainting, then a proper curing
time should be allowed to be sure the acrylics are completely
dry. For thin films on canvas, this can be 1 to 3 days. Thicker
films on less porous supports like masonite may take several weeks
to dry sufficiently to ensure proper adhesion.
For varnishing, it is
best to wait until a painting is completely dry. Otherwise, an
excessive level of water and/or solvent may be trapped, which
could result in clouding or poor initial adhesion of the varnish.
This is more important when the varnish forms a relatively tight,
non-permeable film, such as is the case with GOLDEN MSA Varnish.
When using more permeable varnishes, such as GOLDEN Polymer Varnish,
this is less crucial.
Another critical element to consider
when varnishing is the substrate. A canvas will allow trapped
moisture, retarders and other additives to escape, while metal,
masonite or plywood make this migration difficult, and can influence
adhesion of the varnish layer. In extreme cases, it can cause
delamination or cracking of the varnish. (Refer to the GOLDEN
Varnish Information Sheet for a more through understanding of
these properties). Generally, the painting should not feel cool,
excessively soft or tacky. All of these symptoms indicate an uncured
acrylic paint film.
STORING AND SHIPPING
Generally, a painting
should not be put into any closed-in storage environment until
fully dry. The preferred conditions for paintings that are in
the process of drying is to have ample air flow over the front
and back of the paintings, while experiencing no extremes in temperature
If a painting is rolled while still
curing (a poor storage choice in any event), it will coalesce
as a rolled film. Upon unrolling, it will be curved and in lower
temperatures be more likely to crack.
Shipping Artwork Considerations
When shipping artwork,
it is important to not allow anything other than air to touch
the paint surface. This includes any type of glassine, paper,
bubble wrap or other plastic, as these may stick to the surface
and result in ferrotyping (transferring a texture to the paint
Conservators take many precautions
when shipping artwork in regard to temperature and humidity. After
packing the artwork, allow it time to adapt to its new surroundings.
Acrylics shrink in colder temperatures and swell in warmer temperatures,
so it is critical not to roll, unroll, or excessively disturb
the painting during this transitional period. On flexible supports,
carefully back the work with cardboard to reduce the "bouncing"
the artwork will encounter during shipment.
If the work is going from room
temperature to below freezing temperatures, it is wise to put
the packed painting in a lower temperature environment, i.e. 32-40oF,
for several hours before taking it outdoors. This will lessen
the stressful shock to the acrylic.
Conversely, the same rules apply
after the painting is received. Do not immediately unpack the
piece. Allow it the same chance to re-adjust to warmer temperatures.
If it was subjected to freezing temperatures, put it in an area
of lower temperature as was done before shipping.
SLOWER DRYING TECHNIQUES
Depending on their objectives and
techniques, artists have varying needs to keep paint from drying
too quickly. They may require longer mixing time on the pallet,
or longer blending time on the canvas or other support.
Studio Environmental Conditions
There are as many ways
to control the drying process as there are ways to control the
evaporation of water. High humidity, low temperature, low air
movement and non-absorbent surfaces slow the drying of acrylic
paint. Conversely, dry conditions, high temperatures, significant
air flow and absorbent surfaces speed the evaporation (or absorption)
of water, and therefore also speed the drying process.
By reducing studio air-flow, lowering
temperature, and increasing humidity, an artist can slow down
the drying of acrylic paints without additives or altering painting
methods. Keep in mind that many studios are under-ventilated to
begin with, and it may be better to just be sure there isn't air
directly blowing on the painting while working. It is important
to ensure ammonia and other escaping paint additives are not building
up in the studio space.
Some situations call
for chemical agents that retard the drying of acrylic paint. These
are usually a blend of several materials, predominantly glycols.
By evaporating far more slowly than water, glycols keep the polymer
spheres apart, preventing early coalescence. Humectants, (agents,
such as glycerin, that absorb or hold water), also have been added
to retard or slow drying. However, humectants must be used with
extreme care, as they tend to percolate to the surface of the
film during drying leaving a residue that may reduce inter-coat
adhesion. Retarders are not very effective if paint must stay
wet in thin films on very absorbent surfaces.
Paint additives alone will most
likely not be effective to slow down the drying process. They
must be accompanied by proper environmental factors, a working
support with low absorbency and other contributing factors.
Drying on the Palette
In addition to additives,
several techniques will help keep paints wet on the palette. A
most important first step is to use a non-absorbent palette. Glass
works nicely, as do plastics such as polyethylene. Glass stays
fresh and free from stains, cleans easily and works well as long
as it doesn't break. The problem with a glass palette tends to
be the weight. A new commercial tempered-glass palette made in
several sizes has convenient hand-sized holes for carrying. Many
artists construct their own glass-topped taboret to use with acrylic
paints. On wheels, the glass palette is portable and the artist
can use larger plates of glass without worrying about excess weight
To slow the drying of paints on
glass or plastic palettes, use a small amount of retarder. About
three to ten percent of GOLDEN Retarder will keep a mass of paint
from forming a skin for up to six hours, depending on the atmosphere.
Using excessive quantities of retarder, especially when working
in thicker impasto, will leave the paint skin feeling like soft
gum, as the glycol may not totally release from the film.
Some people prefer to lightly mist
the paint on their palettes. This can be a very effective technique
to provide more open time. A simple plant mister, easily obtained
at a hardware store, may be filled with water (we recommend distilled
or de-ionized) or a 10:1 water/retarder mixture for this purpose.
Note again that excessive levels of retarder will result in a
weakened paint film that remains sticky. Excessive spraying may
cause colors to drip, or to stain the support in an undesirable
On the market are several
covered varieties of moisture-retaining palettes that help keep
acrylic paints in working condition. The most effective is the
Stay-Wet Palette®, which takes advantage of what we know about
acrylic drying: that is, if one can control the atmosphere, they
control the acrylic. The Stay-Wet Palette® is a large plastic
tray on which a large, flat sponge is placed. After soaking the
sponge in water, a special filter paper is placed over it, and
the paper acts as a palette surface. The filter paper allows water
vapor to permeate, keeping the paints moist. If the sponge stays
wet, paints will remain wet for hours without the use of a retarder.
This palette has a cover to further extend working time. Some
artists have suggested that this tool slightly dilutes acrylic
colors, but if you plan to work outdoors with acrylics on a palette,
it may be of great benefit.
When paint dries on a glass or
plastic palette, the best way for removal is to saturate the dried
surface with water, using a very wet sponge or rag. After a period
of 3-5 minutes, the paint should be softened and can easily be
scraped or peeled off.
Slowing Drying on the Substrate
Canvas and other substrates
should be sealed to reduce absorbency, which will aid in keeping
acrylic paints wet on the surface. Several years ago an artist
called us, complaining that our Heavy Body Acrylic was drying
too quickly on her paper. When she first encountered the problem,
she decided to slow down the drying by adding more water to the
paint. Intuitively, her approach made sense, but when she added
water, the paint dried even more quickly. In this case, the drying
was not dictated by evaporation as much as by absorption into
the substrate (paper). The paper was not sealed in any way. As
she thinned the paint, it had less hold-out and the water was
absorbed more easily into the paper. Had she sealed the surface
first with GOLDEN Polymer Medium or a skim coat of GOLDEN Soft
Gel Gloss, the substrate would not have absorbed as much water,
and drying would have occurred mostly by evaporation.
A cool air humidifier
may prove useful to increase the localized relative humidity,
and thus slow the drying process. Directing the cool moisture
flow on the painting surface will maximize the effect. A cheaper
way to achieve a similar result is to use a plant mister set on
a very light spray setting. Spraying across the surface at regular
intervals will dramatically lengthen the wet time of the paint.
Soaking the Back of the Canvas
Another technique to
retard drying capitalizes on the permeability of acrylic gesso.
After allowing the gesso layers to completely dry, attach the
canvas to a temporary stretcher. Soak the back of the canvas with
water, and/or attach wet rags or sponges to the back of the canvas.
You have now created a stay-wet canvas. The dried gesso acts as
a semi-permeable membrane, allowing water through to keep the
paint moist. This technique allows you to paint in relatively
thin glazes for hours with acrylics, but beware of some limitations.
Cotton canvas may shrink, causing the stretcher to warp and make
restretching a necessity. Also, the water may wash impurities
out of the canvas and into the paint film. In severe cases this
causes noticeable discoloration. These problems will be minimized
or eliminated by using scoured (washed) cotton canvas. Or use
polyester canvas, which is not dimensionally sensitive to water
and does not contain the contaminants found in unwashed cotton
To prevent paint from drying out
in storage, be sure not to place it near blowers or heaters. All
plastic containers are slightly porous and allow a small amount
of water vapor to escape. Also, it is critical to clean the threads
of lids and jars. If paint accumulates on these surfaces, the
tops will not seal properly and the paints will begin to dry out.
Some artists suggest spraying a small amount of water on top of
the paint to keep the paint fresh. When spraying, use distilled
or de-ionized water to avoid contaminating the paint. Paints will
stay quite well in most basements, but should not be frozen. Although
most acrylic paints will sustain several freeze-thaw cycles, freezing
is not advisable.
CLEANING ART TOOLS
Tools should be kept wet to keep
paint from hardening on them.
- Avoid storing brushes with the
- Even a small amount of acrylic
building up over time will get into the base of the ferrule
and start to harden the brush.
- Keep your brushes well-conditioned.
Normal hair conditioner works as well as any product marketed
for brushes. This will help reduce the amount of acrylic that
- A small amount of dish detergent
added to water helps remove semi-dried paint from tools.
- For hardened paint, most of
the commercially available brush cleaners will work well, as
long as the brush has not been abused.
- Pre-conditioning brushes with
a small amount of conditioner will greatly improve the clean
up of your brushes. After cleaning a brush, pat the bristles
dry and condition with a small amount of conditioner. Brush
the excess onto a hand until you cannot see it.