Paint,Wood,&Weather
Content
DIRECTORY
ARTICLES
BOOKSHOP
WHAT'S ON
Paint, Wood and Weather
Colin Mitchell-Rose
Wood is one of the oldest and most versatile building materials used by man. It is incredibly strong and can easily be
shaped or carved and joined into elaborate structures. At the same time, it can also be very flexible: it can move with the
wind and even absorb earth tremors. Wood has a great natural beauty which can be enhanced by polishing and
varnishing; it can be sliced into thin layers of veneer that conceal humbler (and cheaper) types of wood. It is easily
obtainable and can be harvested and modelled using the
most simple of tools. And to modern man it has the important
advantage of being a renewable resource and easily
recyclable: a truly 'green' building material.
But wood does have one fault; it can rot. If water penetration
raises the moisture content above a critical level (around 20 per
cent), the timber can be attacked by various moulds and fungi
and start to rot: it is, after all, biodegradable. There are also many types of beetles and their grubs that can weaken and
destroy the integrity of wood if its moisture levels are suitable. Furthermore, wood will also gradually degrade when it is
exposed to sunlight. This is because one of its main components, cellulose, undergoes auto-oxidation in the presence of
UV radiation in sunlight, leading to surface bleaching. (Cellulose itself does not absorb UV, but lignin, hemi-celluloses and
some dyes and pigments cause cellulose to deteriorate because they act as photo-sensitisers, absorbing UV radiation
and transferring the energy. As a result some of the long molecular chains of cellulose break up, weakening the material.)
As this only occurs at the surface it generally does not affect the structural integrity of the timber (see 'The Effects of
Daylight' by Rebecca Ellison in The Building Conservation Directory 2000).
There are many ways of dealing with these challenges, but this article will only deal with the role that paint (in the widest
sense) has in preserving exterior woodwork.
EARLY TREATMENTS
If wood is allowed to dry out and 'season' properly before use, it gains a marked degree of protection against rain and the
elements, particularly if it does not remain damp for long. Seasoned oak, for example, will weather to an attractive silvergrey appearance and, in the right conditions, will remain durable without being painted.
One of the earliest ways of protecting the exposed timber frame of a building from the elements was to coat it with the
same material as that used on the panels between, usually a limewash or colourwash. Limewash is a mixture of fat (nonhydraulic) lime in water, usually with a binder such as tallow. It was often coloured with earth pigments such as ochre,
when it is sometimes referred to as a colourwash. However, the term 'colourwash' is also used to describe a blend of
earth pigments such as ochre, bound with animal glue size and stale beer (see 'Colour Washing and Pencilling of Historic
English Brickwork' by Gerard Lynch in the Journal of Architectural Conservation, July 2006). While neither of these would
last long on timber, applying them regularly would give some protection to the wood and help fill any cracks or gaps
between the timber frame and the panels. Often the timber was either left untreated or the limewash was partially
brushed off, leaving the grain of the timber exposed.
Later it became fashionable to treat the exposed timber frame with liquids from coal tar deposits, the forerunner of
creosote. This black and white effect was noted (and deplored) by a traveller to the Welsh Marches in 1790. It later
spread widely through Victorian England and became known in the 1920s as 'Stockbroker Tudor'.
OIL AND STAINS
Some hardwoods, such as oak and chestnut, are naturally more durable than others, while some softwoods, such as
western red cedar and larch, contain high levels of oils which act as a defence, so they weather to a grey appearance
without harm. Wood treatments like teak oil can be used to replace the oil lost from the wood through weathering and so
help to retain the original colour. Other materials like wax or fungicides can be added, as in the 'Madison formula' which
was widely used on cedar cladding and other timber from the early 1950s. Creosote, which was also found to have
excellent preservative and water-repelling properties, was a mixture of substances obtained from the distillation of coal
tar, and was widely used industrially for many years for preserving fences, railway sleepers and telegraph poles.
Unfortunately these properties made it dangerous to handle and creosote has recently been listed as a carcinogen and
its non-industrial use banned. There are modern 'safe' alternatives that trade upon its name.
Wood stain is, in essence, a modern development of timber oil. The difference between stains and paints or varnishes is
that stains are applied in thin layers that soak into the wood, rather than forming a film on the surface. There are many
types of stain available but they usually contain a thin oil or resin that is absorbed by the wood, a water repellent such as
silicone or wax, pigmentation to keep out UV radiation and a biocide to retard timber decay. Most stains have good
'microporosity', meaning that they allow the wood below to 'breathe' by letting water vapour escape through them.
VARNISHES
If a hard natural resin such as copal or rosin is boiled up with linseed oil, the result is a varnish. This has several cosmetic
advantages over plain oil as it dries to a clear, hard, glossy finish which enhances the appearance of the wood, prevents
water soaking into it and protects the surface from damage. However, there is still one important drawback: UV radiation
will pass through the clear film and attack the surface of the wood. This causes the varnish film to loose adhesion and it
soon starts to crack and flake, leaving the grey 'dead' wood exposed. If pigments are added to the varnish they will reflect
some of the radiation, but at the cost of changing the appearance of the wood: after several applications the wood can
appear a dark muddy brown. Recently it has been found that very fine particles can reflect the radiation without affecting
the transparency of the varnish. These are called UV absorbers and are used in better-quality exterior varnishes,
marketed as yacht, spar or marine varnish.
Of course, the best way to prevent UV attack of wood is to add enough pigment to make the varnish completely opaque;
in other words, a paint. This forms a hard opaque film that should adhere firmly to the wood and also move and flex with
it. It should have a certain degree of microporosity to allow water vapour
to move in and out of the wood, while preventing excess moisture being
absorbed into the wood.
WHITE LEAD PAINTS
It was a happy coincidence that when the most widely available white
pigment, white lead (lead carbonate), was added to the most widely
available drying oil, linseed oil, the two dried together to form a wonderfully
flexible adhesive film. This is because white lead reacts with the linseed oil
to form very flexible compounds which, chemically, are known as 'soaps'.
These made paint based on white lead and linseed oil (usually with small
additions of turpentine and driers) extremely successful for coating and
protecting exterior timber. Rather than becoming brittle and cracking, the
paint would gradually weather or 'chalk' away to a matt surface which could
be easily recoated after a few years.
However, these paints dried slowly and developing tastes also demanded
shinier paints. One solution was to use a varnish rather than just linseed oil
as the binder for the paint. The so-called 'enamel' paints which this produced
had a harder, glossy finish, but as they aged the chemical reactions which
made them dry in the first place continued, making these paints brittle. When
the wood flexed, the film cracked and flaked exposing the wood to the
elements. Many modern alkyd resin-based gloss paints suffer from the same
problems: great initial gloss but poor long-term protection.
Whilst the wood here has initially been protected from UV
radiation by the opaque paint film, the modern gloss paint
has become too brittle to cope with the slight movement of
the wood and has cracked along the lines of the wood
grain. The resulting curling back of the paint, caused by
tension in the paint film, exposes the wood to UV radiation
and loss of adhesion and flaking soon follows.
UV radiation has penetrated the clear varnish and
attacked the surface of this wood, causing it to go grey.
The resulting loss of adhesion has led to the widespread
flaking of the varnish coat.
Although white lead was a very useful white pigment, it was also long known to be toxic, causing long-term chronic
illnesses. Many attempts to find alternatives were made, not always with success, and by the mid 19th century zinc oxide
was being widely used in white paints. However, for exterior paints it was less durable than white lead because, when it
dried, its paint film was less flexible. A compromise which became very popular was a lead/zinc blend, as the zinc oxide
was noticeably whiter than white lead but the film still retained good flexibility.
Titanium dioxide, a very white and non-toxic white pigment became available in the 1930s. The early version was based
on anatase (one of three mineral forms of titanium dioxide) and was not ideal for outside use because it was very prone
to chalking. Later versions based on the rutile form of the mineral were very much better and are now the industry
standard for white paints.
While white pigments tended to grab the attention of chemists, other pigments were being developed. The traditional
earth colours, ochre, umber and sienna, gave good coverage and hence protection to timber, and were also light-fast.
Many brighter inorganic pigments were developed in the 18th and 19th centuries, but they were based on heavy metals
such as lead, mercury, arsenic and copper and so had similar toxicity problems to white lead. The late 19th century saw
the development of synthetic pigments and dyes made from petrochemicals, but early versions tended to fade in sunlight
and so lose their UV protection. Today, most pigments used for tinting or colouring paints are synthetic organic pigments
but have much better light fastness.
MODERN ALTERNATIVES
With so many 'traditional' paints or their ingredients no longer available today, what materials can a specifier use for
conservation work? White lead paints are still available, but only for listed buildings of Grades I and II* in England and
Wales, and grade A in Scotland, and formal approval has to be sought from English Heritage, Cadw or Historic Scotland.
There are only a few suppliers left and the major UK paint makers are exerting pressure on government to rescind the
present derogation for the use of white lead paints on historic buildings. It is very much a case of 'use it or lose it' and the
outstanding properties, appearance and historical authenticity of white lead paints still make them first choice where they
can be safely used.
For lead-free alternatives, several types of linseed-oil paints are available (most imported from Scandinavia) which have
a long history of successful use in timber preservation. It is most important that the linseed oils used in the paint are
suitably treated to avoid being attacked by mildew. Whilst the application of these paints can be time-consuming,
particularly when older layers of 'modern' paints need to be removed, they will give an appreciably longer life, as well as
weathering to an attractive appearance.
Another modern alternative is to use a water-borne acrylic paint. These paints can be glossy or matt and, being water
based, release no harmful solvents. (Volatile organic compounds, or 'VOCs', can contribute to atmospheric pollution.)
Other advantages include their non-yellowing, quick-drying properties, ease of application, gloss retention and most
importantly, no loss of flexibility with aging. With such an impressive list of attributes, you might well wonder why they are
not more widely used. This may well be due to suspicion of any new material, although they first appeared on the market
over 35 years ago. The early examples performed extremely well, and there have been many technical improvements
since then, but like any advanced paint, they perform best when applied to a clean surface, free from old failing paint
layers, and this is not always possible. Water-borne acrylic paints last extremely well outside: they have even been used
for painting television masts in such exposed locations as mountain tops with great success.
GENERAL PRINCIPLES
The first step in the choice of a paint system for protecting external timber is to decide what the final appearance of the
timber should be, and then to select the appropriate coating: oil, stain, varnish, semi-transparent, opaque, matt or glossy.
Whichever is chosen, there are a few principles that must be followed to achieve the best possible solution and avoid
premature failure:
New wood should be of the best available quality and should have been allowed to season properly
Old wood must be sound, and any rot or decay treated before painting
Preparation is the key to success: seek advice from the coating supplier as to what is required and ensure your
contractor carries it out
Whatever coating you choose, make sure that it will remain flexible and has adequate microporosity
Establish a regular inspection and maintenance regime: a 'stitch in time' will save heavy repair costs later
Coatings & Finishes
Contents
Coatings & Finishes
Protective Coated Pine
Once placed in service timber should be coated to enhance the appearance and/or the durability of the material.
Interior timber surfaces are easily and satisfactorily coated with a wide range of products such as paints, stains or oils as the timber is normally seasoned and
free of surface impurities which hamper the coating process.
Exterior timber surfaces on the other hand are exposed to the elements of ultraviolet light, heat, cold, water and airborne contaminates before and during coating
and therefore create difficulties for the painter. The purpose of an exterior coating is firstly to provide a protective coating against weathering and secondly to
provide an appealing overall finish to the structure.
The selection of the correct coating product is essential to maintain visual appeal and also for the ongoing performance of the timber. Each timber species has
particular characteristics that could effect the application of certain products and it is important to know a little about the timber being coated and make reference
to the various manufacturers' product literature for specific recommendations.
When any timber (treated or untreated) is exposed to the weather, the timber will take up and lose moisture from or to the atmosphere as the environment around
the timber changes on daily or seasonal basis. The timber will expand or contract when it takes up or loses moisture.
As a result of taking up and losing moisture, the timber will develop surface checking. The severity of the checking will depend on the timber species, the rate at
which the moisture content of the timber changes and how long that moisture content is maintained before it changes again.
The application of a protective coating to the timber surface will minimise the effects of weathering of any timber (treated or untreated) in an exposed situation.
The protective coating slows down the rate at which the timber will take up or lose moisture. By slowing that rate down, the severity of any checking is
considerably reduced.
Protective coatings include products which penetrate the surface of the timber and products which provide a film or coating to the surface of the timber.
Protective coated pine is widely available on the Australian market in a broad range of accurately sized and milled products for a variety of external, above ground
applications. (See
DIRECTORY
ARTICLES
BOOKSHOP
WHAT'S ON
Paint, Wood and Weather
Colin Mitchell-Rose
Wood is one of the oldest and most versatile building materials used by man. It is incredibly strong and can easily be
shaped or carved and joined into elaborate structures. At the same time, it can also be very flexible: it can move with the
wind and even absorb earth tremors. Wood has a great natural beauty which can be enhanced by polishing and
varnishing; it can be sliced into thin layers of veneer that conceal humbler (and cheaper) types of wood. It is easily
obtainable and can be harvested and modelled using the
most simple of tools. And to modern man it has the important
advantage of being a renewable resource and easily
recyclable: a truly 'green' building material.
But wood does have one fault; it can rot. If water penetration
raises the moisture content above a critical level (around 20 per
cent), the timber can be attacked by various moulds and fungi
and start to rot: it is, after all, biodegradable. There are also many types of beetles and their grubs that can weaken and
destroy the integrity of wood if its moisture levels are suitable. Furthermore, wood will also gradually degrade when it is
exposed to sunlight. This is because one of its main components, cellulose, undergoes auto-oxidation in the presence of
UV radiation in sunlight, leading to surface bleaching. (Cellulose itself does not absorb UV, but lignin, hemi-celluloses and
some dyes and pigments cause cellulose to deteriorate because they act as photo-sensitisers, absorbing UV radiation
and transferring the energy. As a result some of the long molecular chains of cellulose break up, weakening the material.)
As this only occurs at the surface it generally does not affect the structural integrity of the timber (see 'The Effects of
Daylight' by Rebecca Ellison in The Building Conservation Directory 2000).
There are many ways of dealing with these challenges, but this article will only deal with the role that paint (in the widest
sense) has in preserving exterior woodwork.
EARLY TREATMENTS
If wood is allowed to dry out and 'season' properly before use, it gains a marked degree of protection against rain and the
elements, particularly if it does not remain damp for long. Seasoned oak, for example, will weather to an attractive silvergrey appearance and, in the right conditions, will remain durable without being painted.
One of the earliest ways of protecting the exposed timber frame of a building from the elements was to coat it with the
same material as that used on the panels between, usually a limewash or colourwash. Limewash is a mixture of fat (nonhydraulic) lime in water, usually with a binder such as tallow. It was often coloured with earth pigments such as ochre,
when it is sometimes referred to as a colourwash. However, the term 'colourwash' is also used to describe a blend of
earth pigments such as ochre, bound with animal glue size and stale beer (see 'Colour Washing and Pencilling of Historic
English Brickwork' by Gerard Lynch in the Journal of Architectural Conservation, July 2006). While neither of these would
last long on timber, applying them regularly would give some protection to the wood and help fill any cracks or gaps
between the timber frame and the panels. Often the timber was either left untreated or the limewash was partially
brushed off, leaving the grain of the timber exposed.
Later it became fashionable to treat the exposed timber frame with liquids from coal tar deposits, the forerunner of
creosote. This black and white effect was noted (and deplored) by a traveller to the Welsh Marches in 1790. It later
spread widely through Victorian England and became known in the 1920s as 'Stockbroker Tudor'.
OIL AND STAINS
Some hardwoods, such as oak and chestnut, are naturally more durable than others, while some softwoods, such as
western red cedar and larch, contain high levels of oils which act as a defence, so they weather to a grey appearance
without harm. Wood treatments like teak oil can be used to replace the oil lost from the wood through weathering and so
help to retain the original colour. Other materials like wax or fungicides can be added, as in the 'Madison formula' which
was widely used on cedar cladding and other timber from the early 1950s. Creosote, which was also found to have
excellent preservative and water-repelling properties, was a mixture of substances obtained from the distillation of coal
tar, and was widely used industrially for many years for preserving fences, railway sleepers and telegraph poles.
Unfortunately these properties made it dangerous to handle and creosote has recently been listed as a carcinogen and
its non-industrial use banned. There are modern 'safe' alternatives that trade upon its name.
Wood stain is, in essence, a modern development of timber oil. The difference between stains and paints or varnishes is
that stains are applied in thin layers that soak into the wood, rather than forming a film on the surface. There are many
types of stain available but they usually contain a thin oil or resin that is absorbed by the wood, a water repellent such as
silicone or wax, pigmentation to keep out UV radiation and a biocide to retard timber decay. Most stains have good
'microporosity', meaning that they allow the wood below to 'breathe' by letting water vapour escape through them.
VARNISHES
If a hard natural resin such as copal or rosin is boiled up with linseed oil, the result is a varnish. This has several cosmetic
advantages over plain oil as it dries to a clear, hard, glossy finish which enhances the appearance of the wood, prevents
water soaking into it and protects the surface from damage. However, there is still one important drawback: UV radiation
will pass through the clear film and attack the surface of the wood. This causes the varnish film to loose adhesion and it
soon starts to crack and flake, leaving the grey 'dead' wood exposed. If pigments are added to the varnish they will reflect
some of the radiation, but at the cost of changing the appearance of the wood: after several applications the wood can
appear a dark muddy brown. Recently it has been found that very fine particles can reflect the radiation without affecting
the transparency of the varnish. These are called UV absorbers and are used in better-quality exterior varnishes,
marketed as yacht, spar or marine varnish.
Of course, the best way to prevent UV attack of wood is to add enough pigment to make the varnish completely opaque;
in other words, a paint. This forms a hard opaque film that should adhere firmly to the wood and also move and flex with
it. It should have a certain degree of microporosity to allow water vapour
to move in and out of the wood, while preventing excess moisture being
absorbed into the wood.
WHITE LEAD PAINTS
It was a happy coincidence that when the most widely available white
pigment, white lead (lead carbonate), was added to the most widely
available drying oil, linseed oil, the two dried together to form a wonderfully
flexible adhesive film. This is because white lead reacts with the linseed oil
to form very flexible compounds which, chemically, are known as 'soaps'.
These made paint based on white lead and linseed oil (usually with small
additions of turpentine and driers) extremely successful for coating and
protecting exterior timber. Rather than becoming brittle and cracking, the
paint would gradually weather or 'chalk' away to a matt surface which could
be easily recoated after a few years.
However, these paints dried slowly and developing tastes also demanded
shinier paints. One solution was to use a varnish rather than just linseed oil
as the binder for the paint. The so-called 'enamel' paints which this produced
had a harder, glossy finish, but as they aged the chemical reactions which
made them dry in the first place continued, making these paints brittle. When
the wood flexed, the film cracked and flaked exposing the wood to the
elements. Many modern alkyd resin-based gloss paints suffer from the same
problems: great initial gloss but poor long-term protection.
Whilst the wood here has initially been protected from UV
radiation by the opaque paint film, the modern gloss paint
has become too brittle to cope with the slight movement of
the wood and has cracked along the lines of the wood
grain. The resulting curling back of the paint, caused by
tension in the paint film, exposes the wood to UV radiation
and loss of adhesion and flaking soon follows.
UV radiation has penetrated the clear varnish and
attacked the surface of this wood, causing it to go grey.
The resulting loss of adhesion has led to the widespread
flaking of the varnish coat.
Although white lead was a very useful white pigment, it was also long known to be toxic, causing long-term chronic
illnesses. Many attempts to find alternatives were made, not always with success, and by the mid 19th century zinc oxide
was being widely used in white paints. However, for exterior paints it was less durable than white lead because, when it
dried, its paint film was less flexible. A compromise which became very popular was a lead/zinc blend, as the zinc oxide
was noticeably whiter than white lead but the film still retained good flexibility.
Titanium dioxide, a very white and non-toxic white pigment became available in the 1930s. The early version was based
on anatase (one of three mineral forms of titanium dioxide) and was not ideal for outside use because it was very prone
to chalking. Later versions based on the rutile form of the mineral were very much better and are now the industry
standard for white paints.
While white pigments tended to grab the attention of chemists, other pigments were being developed. The traditional
earth colours, ochre, umber and sienna, gave good coverage and hence protection to timber, and were also light-fast.
Many brighter inorganic pigments were developed in the 18th and 19th centuries, but they were based on heavy metals
such as lead, mercury, arsenic and copper and so had similar toxicity problems to white lead. The late 19th century saw
the development of synthetic pigments and dyes made from petrochemicals, but early versions tended to fade in sunlight
and so lose their UV protection. Today, most pigments used for tinting or colouring paints are synthetic organic pigments
but have much better light fastness.
MODERN ALTERNATIVES
With so many 'traditional' paints or their ingredients no longer available today, what materials can a specifier use for
conservation work? White lead paints are still available, but only for listed buildings of Grades I and II* in England and
Wales, and grade A in Scotland, and formal approval has to be sought from English Heritage, Cadw or Historic Scotland.
There are only a few suppliers left and the major UK paint makers are exerting pressure on government to rescind the
present derogation for the use of white lead paints on historic buildings. It is very much a case of 'use it or lose it' and the
outstanding properties, appearance and historical authenticity of white lead paints still make them first choice where they
can be safely used.
For lead-free alternatives, several types of linseed-oil paints are available (most imported from Scandinavia) which have
a long history of successful use in timber preservation. It is most important that the linseed oils used in the paint are
suitably treated to avoid being attacked by mildew. Whilst the application of these paints can be time-consuming,
particularly when older layers of 'modern' paints need to be removed, they will give an appreciably longer life, as well as
weathering to an attractive appearance.
Another modern alternative is to use a water-borne acrylic paint. These paints can be glossy or matt and, being water
based, release no harmful solvents. (Volatile organic compounds, or 'VOCs', can contribute to atmospheric pollution.)
Other advantages include their non-yellowing, quick-drying properties, ease of application, gloss retention and most
importantly, no loss of flexibility with aging. With such an impressive list of attributes, you might well wonder why they are
not more widely used. This may well be due to suspicion of any new material, although they first appeared on the market
over 35 years ago. The early examples performed extremely well, and there have been many technical improvements
since then, but like any advanced paint, they perform best when applied to a clean surface, free from old failing paint
layers, and this is not always possible. Water-borne acrylic paints last extremely well outside: they have even been used
for painting television masts in such exposed locations as mountain tops with great success.
GENERAL PRINCIPLES
The first step in the choice of a paint system for protecting external timber is to decide what the final appearance of the
timber should be, and then to select the appropriate coating: oil, stain, varnish, semi-transparent, opaque, matt or glossy.
Whichever is chosen, there are a few principles that must be followed to achieve the best possible solution and avoid
premature failure:
New wood should be of the best available quality and should have been allowed to season properly
Old wood must be sound, and any rot or decay treated before painting
Preparation is the key to success: seek advice from the coating supplier as to what is required and ensure your
contractor carries it out
Whatever coating you choose, make sure that it will remain flexible and has adequate microporosity
Establish a regular inspection and maintenance regime: a 'stitch in time' will save heavy repair costs later
Coatings & Finishes
Contents
Coatings & Finishes
Protective Coated Pine
Once placed in service timber should be coated to enhance the appearance and/or the durability of the material.
Interior timber surfaces are easily and satisfactorily coated with a wide range of products such as paints, stains or oils as the timber is normally seasoned and
free of surface impurities which hamper the coating process.
Exterior timber surfaces on the other hand are exposed to the elements of ultraviolet light, heat, cold, water and airborne contaminates before and during coating
and therefore create difficulties for the painter. The purpose of an exterior coating is firstly to provide a protective coating against weathering and secondly to
provide an appealing overall finish to the structure.
The selection of the correct coating product is essential to maintain visual appeal and also for the ongoing performance of the timber. Each timber species has
particular characteristics that could effect the application of certain products and it is important to know a little about the timber being coated and make reference
to the various manufacturers' product literature for specific recommendations.
When any timber (treated or untreated) is exposed to the weather, the timber will take up and lose moisture from or to the atmosphere as the environment around
the timber changes on daily or seasonal basis. The timber will expand or contract when it takes up or loses moisture.
As a result of taking up and losing moisture, the timber will develop surface checking. The severity of the checking will depend on the timber species, the rate at
which the moisture content of the timber changes and how long that moisture content is maintained before it changes again.
The application of a protective coating to the timber surface will minimise the effects of weathering of any timber (treated or untreated) in an exposed situation.
The protective coating slows down the rate at which the timber will take up or lose moisture. By slowing that rate down, the severity of any checking is
considerably reduced.
Protective coatings include products which penetrate the surface of the timber and products which provide a film or coating to the surface of the timber.
Protective coated pine is widely available on the Australian market in a broad range of accurately sized and milled products for a variety of external, above ground
applications. (See
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