Wallcovering Adhesives

by Chris Murphy

"It is well known that it is impossible to make a good adhesive paste of any other than flour from perfectly sound wheat (author's italics)...."

-James Arrowsmith, "The Paper-Hanger's Companion" (1856)

I'm sure all of us have heard that before the invention of modern pre-mixed wallcovering adhesives there was only wheat paste. Indeed, A.A. Kelly said in 1921, "....before the introduction of such wallcoverings as Lincrusta, pressed papers, burlaps, etc. one paste answered for all purposes."1 This was only partly true as both the above-mentioned hanger/authors had at least four types of wheat paste, all using different or no additives. Turns out, those trial-and-error additives have their modern counterparts.

But let's back up for a moment. Why wheat? And while we're at it, how does it do its work as an adhesive? Just what makes an adhesive.....adhesive?

The Egyptians had used wheat flour paste over 3,300 years ago to bond the fibers of reeds to make the paper-like papyrus. And that is the only early use we now have evidence of, it could have been used much earlier. Cooking flour and water was a common additive to foods like soups and stews (as now) so it must have been a common observation that the thickened "white sauce" stuck well wherever it was left to dry. Other starches act much the same, but in Western civilizations wheat was the most common grain, and source of starch. The use of adhesives arose out of the need for fastening systems that spread the force of the attachment out over the surface of the materials being bonded.

The use of adhesives arose out of the need for fastening systems that spread the force of the attachment out over the surface of the materials being bonded. An adhesive is a substance that can hold materials together and that resists separation.2 Adhesives flow into the empty spaces between materials (the interface). The adhesive allows a close fit between the material surfaces (the adherends), promoting a bond between the molecules. "The closer the two surfaces fit together, the stronger the bond, so a thin adhesive bond is stronger than a thick one."3 The interface- the adhesive in contact with neither surface- is the weak point in the bond.

Once the adhesive spreads to the voids or irregularities of the surfaces the mechanisms of adhesion come into play. More than one force may be at work, depending on the specific adhesive and surfaces present. With our trade's pastes, a mechanical bond is the most common. Adhesives flow into pores or surround surface projections (like paper fibers). That is why adding 'tooth' to a surface by sanding will allow a paper to stick. There may also be a chemical bond, where the molecules of the adhesive and adherends interact by valence forces, that is, positive and negative charges attracting each other.4 There are other types of adhesive bonds, but these two- mechanical and chemical- are the ones at work with our pastes and primers.

In former times, hangers had glues (animal-based) and pastes (plant-based). The glues were used as sizes: tacky substances that conditioned the wall surface. They were gelatinized materials made from hides, bones, blood or dairy products (casein).5 Glues were sometimes added to pastes, but they had the propensity to stain. These natural glues are rare these days. Pastes were made from carbohydrate-bearing plants, like wheat, corn, rice or potatoes. These products were, and are, processed to remove impurities and proteins to yield starch. Starches are complex carbohydrates whose molecules have a basic composition of six carbon atoms, ten of hydrogen and five of oxygen (C6 H10 O5- glucose).6 These units are strung together like long chains that are called polymers: repeating sequences of molecules. A starch polymer can have 3,000 Ð 10,000 of these units.

Starches don't mix well with water, but upon heating the mix the granular starch molecules will take on water and swell, a process known as gelatinization. As the mass cools, it will swell more as molecular (chemical) bonding occurs.7 This paste is now useful as an adhesive, as its viscosity (thickness) allows its flow (spreadability) to be controlled, and its polymers are capable of mechanical bonding.

Both Arrowsmith and Kelly used the same basic recipe. Start with 2 lbs. of wheat flour (preferably a hard, winter wheat). Add lukewarm water enough to make a smooth batter, stirring to smooth out lumps. Then gradually add boiling water to make a stiff, doughy mass. This cooks it: when it turns slightly darker, it is done cooking. Stir is some cold water to make it less thick, and smooth. Cover the top of the mix with cold water to keep it from skinning over. When cooled, pour off that water, cut a hunk from the whole and wrap it in paper. At the job. mix with water to desired thickness.8

Remember that the old-timers hung almost exclusively on plaster. This is a porous surface, but generally favorable to paperhanging. But buildings then had no central heat/air, nor insulation behind the plaster. Temperature and moisture extremes were the norm. The wood-pulp papers were especially moisture-sensitive. Hangers had to resort to sizes and additives to overcome these conditions. Glue size had two purposes: it lessened the porosity of the plaster, and it added tack. Arrowsmith, owing to his times (1856), preferred that made from buffalo-hide scraps.9 It could be bought it bar form, and heated in water until it dissolved. Mixed thinly, it was put on warm. Like some of our primers and pastes, glue size and wheat paste became a 'system.' They would be adjusted for viscosity and strength, and then other additives would be mixed in for certain papers and/or wall conditions. The pastes, glues and additives seem to have been tested by on-site trial and error: in desperation, anything in the cupboard or workshop was added to aid the efficacy of a mix. If something worked, it was always informed guess-work as to why it did, but they had great success: come on over and try- just try!- to get the 80-year-old paper off in my sized-plaster house!

The most common additive in paste was alum (aluminum sulfate). From A.A. Kelly: "Alum makes a paste firmer, causes it to spread well, preserves it against decomposition, prevents the paper from becoming moldy on damp walls, and makes a stronger paste. With ingrains (a dyed paper) it is useful in setting the color, and causes the paste to dry out quicker, which with some papers is a distinct advantage. Alum injures metallics, bronzes, and gilts, causing these metals to tarnish, hence it must be omitted.... And as alum hardens the paste it will not do on hard walls (non-porous).... (use of) one ounce to two pounds of flour is the maximum...."10 Crystallized alum was ground into a powder with mortar and pestle.

Rosin- powdered pine sap- an ingredient in varnish, was used on non-porous walls. This had to be heated and melted, and added hot to paste (4 oz. per 2 gal. paste). It gave some body and tack, and resisted moisture. Venice turpentine- turpentine from larch trees, with a greenish cast; one tablespoon to 2 gal. of paste, added hot- would be used for the same reasons. Both were used on prepped varnished walls.

The best paste-strength boosters, by far, were sugars. Molasses, brown sugar, white sugar ("for delicate papers"), honey, you name it. Francis Neal (Atlanta chapter, retired), who hung his first strip in '33, used to add a bottle of Karo Syrup (clear corn syrup) to his pail. All were added to sizes, too. Tack you could feel, for sure.

By now you must be saying, wheat flour? boiled hides? molasses? together? Yeah, buddy, wallpaper was not known as 'sanitary' (until Sanitas, an oil-cloth) and was demonized by some for its attraction to bugs, insect and otherwise. But our fore-bears had answers for that, too. Copper sulfate, known as blue stone was ground for paste; copper deters mold and fungi. Formalin, formaldehyde, borax, boric acid and oil of cloves also worked, making the mix unpalatable or fatal to insects and bacteria.

This was a lot of work! But so it was ever to be until the advent of vinyl wallcovering. In the mid 50's the first vinyls came out, some 50 mils thick (about 1/16"). Adding carpenters' glue helped keep the material up but mildew became a huge problem.11 The glues and additives didn't clean well, and adhesion was not adequate.

Arrowsmith tells of a painter who added clay into his paint and successfully coated a damp wall.12 I do not know if this became a common remedy, nor if those who were trying to solve the vinyl dilemma knew of it. Hangers and paste makers have always been chasing wallcovering manufacturers. I couldn't track down who first added clay to paste, but according to Izzy Gecker (NGPP, retired), "It was manna from heaven. Without clay-based adhesive, we doubt that vinyl could've become today's predominant product of choice."13

The clay, an inert ingredient, replaced some of the water in the starch-based paste (a filler), allowing it to set-up and dry much quicker. They did, and do, use a mineral called kaolin, also known for its use in pottery (refined and bleached: White China Clay) and in glossy papers. It comes in different refined grades, and colors- from light tan to slightly gray. The paste grades receive a minimum of processing, keeping the cost down.14 To us, the great advantage of clay is the 'wet" tack it gives paste. You could mix clay and water alone and hang a fairly heavy vinyl with it- until it dried and collapsed.15

While these first clays were being experimented with, research was started into possible uses of starch products, as the huge agro-industrial giants that produced them wanted to find more markets. Over 130 years before, the Frenchman Anselm Payen had discovered dextrin (1838) and cellulose (1833). Both of these came from those familiar glucose building blocks (C6 H10 O5).16 Dextrin is a gummy substance that is produced by treating a starch with heat, acids and/or enzymes that break down the long-chain polymers. Dextrin and other similarly processed starches were found to have great tack because the smaller molecules provided more bonding points. Price and availability on an industrial scale made corn the choice starch. The food industry was using it and its derivatives extensively as fillers, thickeners and sweeteners. Starch adhesives mainly found uses in the paper and paperboard industries. Synthetic and rubber adhesives shared some of the polymer research for the furniture and plywood industries.

Interestingly, A.A. Kelly and other World War I era hangers had used corn starch as a paste when war needs made wheat scarce (The Allied Wall Paper Industry gave out recipes). Kelly liked it but could only obtain it in 1 lb. bags so he went back to wheat when he could.17

There was still the mold and mildew problem to contend with. Mercury had been used up to the mid '80's as a preservative. There was never enough in pastes to be harmful, but the general trend in all industry was to find alternatives. Eventually environmental regulations made it difficult to use, but alternative materials also meant new machines and methods had to be adopted. For instance, hoses and other production surfaces had to be cleaned much more frequently. The biocides and fungicides now used are very high cost (well over $100/gal.), making it expensive to preserve a cheap product. 18 These substances are used in very low concentrations, but don't pour them on your wheat-paste pancakes. There is only enough preservative to keep the paste 'fresh' for about a year, and when a can is opened spores and microorganisms in the air and introduced water immediately start to attack it. Pour paste into a separate clean bucket to work out of, and don't put paste you have been working with (like from a machine) back into the unused portion.

Other ingredients control the flow, drying time and viscosity. Of course water is usually, but not always, the largest percentage ingredient; part may be bound-up with other ingredients. Sodium nitrate and urea- both salts- and propylene glycol help make the paste flow and add drying time. Calcium carbonate and other powders add solids. Corn syrup and other sugars add to the wet tack. Cellulose may also be used to hold water and help the paste have a smooth body.

One paste is still made with wheat, and it claimed to be the first 'clear' on the market: Golden Harvest 34 (now made by Roman Adhesives).19 Clear is a marketing, not a descriptive term; they dry 'clearer' than clay. Aside from GH-34, the rest use corn starches that vary because of specific processing (34's wheat starch is also converted to smaller molecules by heat, acids and enzymes). One of the great pluses of this starch science is that some are made so that when they dry (cure), they crystallize. With a shearing force, like pulling at the face of fabric-backed vinyl, they fracture in the paste layer, making them "strippable."

The clears are not as strong as the clays with a solids content of 20-35%. The clay with the highest solids is 50%, and most over 40%. Clears generally work better in pasting machines, although there are machines that can handle clay. Clears seem to have an advantage in that many claim to be 'non-staining.' Again, this seems to be a marketing term, as any hanger knows a mess can be made with any paste, that technique is more important than any specific paste in regard to staining. Clears dry to a hazy opacity, not clear.

I've only mentioned cellulose and its molecular makeup, which has the same elements as the starches but in a different configuration. Wood or plant fibers are chemically treated to extract cellulose. It is modified to make it a cold-water mix, methyl cellulose. Cellulose adheres much the same as the starches, but it has a greater ability to hold onto water. If there is a non-staining paste, this is it: any remaining on the surface can usually be flaked off. It has the weakest bond of any paste, and only a 3% solids content; the cold-water starches have 7-10%. There is one paste- C-11 from Gardner-Gibson, that looks and acts like cellulose. It has about a 15% solids content, and is made with a polyacrylate, a copolymer. Copolymers are synthetic long-chain molecules that are processed to bond together, giving them some added strength. Cellulose and its
C-11 cousin are hard to wipe clean because of their gel-like nature. Cellulose absorbs water for hours after mixing, making it a good partner with other pastes when you want to control moisture absorption by a paper.

Synthetics chemistry brings us to the white glues of the trade: seam, border and vinyl-over-vinyl adhesives. Using a base of starch adhesives, these use man-made substances like ethylene vinyl acetate (EVA) as the key bonding adhesive. The ability to stick to non-porous substrates is aided by acrylic latex resins, same as our primers.20 They do get their ability to stick to slick vinyl by chemical bonds. Follow the manufacturer's instructions for use as these products are not interchangeable with each other. These pastes do not clean up well, and the VOV's are so thick that they are hard to spread. Mixed in with a premix they are a nightmare to strip as once they harden they cannot be re-moistened.

Polyvinyl acetate (PVA) is listed as an ingredient in many European pastes, used only on vinyls. The hangers there generally use cellulose or powdered potato paste on the rest. Then again, those Old Worlders do a lot of funny things. For instance, many of us have seen the endorsement of 'tub paste' in the hanging directions for European wood-pulp papers. We American hangers, who thought we spoke English, usually assume they mean premix, which comes in a tub, er, pail. Sometimes, not always, but often enough to impede one's financial progress, these pulps will show marks that look wet after the sheets are pasted. The first thought is, "I sure hope that dries out." This darkening can happen anywhere or all over the piece but most often shows at seams and cut edges.

Frank Nicholson of Gardner-Gibson (and NGPP National Associate President) tells me he investigated similar staining some years ago. His apt description of the look of the staining is "parchmentized," meaning it looked like parchment, prepared hides used for writng that have a somewhat translucent appearance. He followed the trail back to the paper substrate manufacturers and found that the stains seemed to occur when melamine resin was not added to the paper mix. This colorless resin is impregnated into papers to give them better wet strength, shrink resistance and printability.21 When the starches used in premixes are processed into smaller molecules, they apparently can 'bleed' through pulps with no melamine barrier. They may also evade the barrier at the seam edges by going in through the exposed side and carried up to the surface by capillary action. More insidious, months or years after an installation the premixes can start to spot and yellow the pulps. One answer, found by South Jersey member Cliff Hayes, is Ecofix P7. This is a Swedish potato paste that reportedly works well in machines.22

The latest thing in pastes are the paper-paste-primer 'systems' now being offered by a few companies. Non-woven wallcoverings made with the new strong, long fibers can be dry hung and dry-stripped if the specified pastes and primers are used. These premixes are weaker than their corporate cousins and my have a disappearing dye added to aid DIY'ers in the paste-the-wall technique. I'm sure that using different products or pasting methods will void some implied warranty that never existed in the first place.

Activators for prepastes main function is to wet-out the prepaste, and add some tack. They can be starches, cellulose or other polymers that hold water. As to prepastes themselves the old powdered starches seem to be rare; they felt like sand on the paper back. Cellulose is common, the swelled goo being unmistakeable. On these, a slightly thinned premix may be best. Cellulose or an activator may work best on the prepastes that need more moisture.

The future of paperhanging is unlikely to change from its earliest days: the paperhanger is handed some goods and must find the best way to get it on the wall. Mixing varied ingredients to achieve successful results has always been a part of the trade. Getting it wrong has also been a part of our "opportunities for growth," so lean on fellow Guild members and our National Associates for advice. There's a solution out there, it just may take someone at the stove to figure it out.