Somewhere out on the Oklahoma prairie, a lonely oil derrick labors in the summer sun, extracting the liquid remains of a brontosaurus. Up, down, up, down, tirelessly it pumps, while hundreds of feet below, precious crude leaps up a steel straw and squirts toward the surface.

It is one more step on a long voyage. Long ago, the fauna and flora of the Jurassic period began generating the elements that would one day become . . . an ordinary pool cover. That’s right. That humdrum piece of fabric you see stretched across a pool in winter was once a razor-fanged, flesh-eating monster, big as a house. Or perhaps it was only an ancient fern.

We’ll never know. It was organic material of one kind or another. In any case, your average pool cover has been on quite an odyssey.

And in order to sell it well, you ought to know something of its story — what it’s composed of, and how it is put together.

Liquid To Solid

Whether made of polypropylene mesh, or vinyl, or another polymer, pool covers are petroleum products. Even the webbing used to stretch the cover tight and hold it in place is petroleum-based.

It’s no accident or convenience of manufacture; just as water does not mix with oil, neither does it mix with pool covers survive the soggy winter, stretched between snow above and ice below. For what water cannot penetrate, it cannot deteriorate. This begs a question — How does gooey, black, liquid oil become the sturdy sentinel of the backyard pool, capable of supporting large animals and heavy snows?

We begin to answer that question at the Ten Cate Nicolon Corp., Jefferson, Ga., one of the major suppliers to the cover industry. In the plant’s receiving bay, enormous bags of what appear to be coffee grounds are being unloaded. On closer inspection these are revealed to be tiny, black polymer granules, about the size of cherry tomato seeds.

The granules are poured into a vat and melted down into a kind of blackplastic lava to which coloring agents and UV- resistant chemicals are added, according to Julie Patty, product manager.

She notes that melting these elements into the base material, thereby diffusing them throughout the goo that will become yarn, gives the material greater durability and color stability than fabrics made with coated yarn. “If that coating breaks down,” she says, “you expose the core yarn.” From there, the molten yarn is ext ruded through a ser ies of spinnerettes or dies, which define its shape. After extrusion, the yarn is cooled and wound on giant spools, each carrying anywhere from 3,000 to 10,000 yards. And then it’s on to the great loom.

“After weaving,” Patty says, “the fabric goes through a two-step process; an oven heat-sets the fabric, pre-shrinks it and straightens out the pick lines. From there it goes into a calender machine, which applies heat and pressure with rollers, bonding the yarns further. That determines the fabric’s permeability to air and water.” All these processes are designed to impart the characteristics pool owners want in a mesh pool cover — UV, chemical and abrasion resistance; strength; and precise permeability to name just a few. And these can be altered to meet new demands from the marketplace — that’s the science of textiles.

For instance, at least one safety cover manufacturer, Meyco Products of Melville, N.Y., decided recently that its polypropylene mesh was too permeable for certain customers, according to Philip Saltzman, director of sales.

The rainwater and melted snow draining through its covers was carrying with it “some fine debris, such as dust and pollen, as well as some light,” he says. “Together these can, under certain circumstances, end up causing photosynthesis and algae, particularly in pools that don’t open until June.”

The company wanted to offer a tighter weave, which would filter out more organic material and admit less sunlight. Working with its fabric supplier, the two were able to adjust the finishing process — where the yarns are heated and pressed together — as well as the denier (the size of the individual threads) and weave to come up with a mesh that blocks all but 1 percent of incidental sunlight. “Basically, it prevents photosynthesis from occurring in the pool,” says Saltzman.

Other character is tics, such as strength, are a matter of fabric design as well, and can be increased if necessary, notes Patty. “A fabric derives its strength in a number of ways. The construction of the fabric in terms of number of yarns per square inch in both the warp and fill direction impact strength, as well as the kind of yarns that you use,” she says.

Tailor Made

With the cover fabric pressed and delivered, the real challenge begins; the material arrives not whole, but in strips. There are no manufacturers that make a single sheet of material wide enough to cover a pool. Such weaving equipment does not exist. So how do you turn these 5- and 6-foot-wide strips of fabric into a tiedown pool cover? For a standard rectangular pool, it’s a relatively straightforward matter of taking measurements of the pool in two dimensions, says Roshan Patel, marketing, The Cover Company, Branchburg, N.J., then laying out the strips at the factory and joining them together.

If it’s a mesh cover, they must be sewn. Manufacturers use specially designed sewing machines with multiple needles and ample space under the arm so the fabric can go through without folding.

The edges of solid-vinyl covers, however, must be fused under heat and pressure. For this, Anchor Industries of Evansville, Ind., uses large radio-frequency welding machines, according to Lincoln Christensen, vice president, Custom Products Division. With the fabric strips in place and under pressure, a giant microwave oven heats the vinyl edges until they melt and fuse together.

But for many manufacturers, this neat rectangular example is generally the exception. The pool to be covered is often not a perfect box, but one with huge bites out of it for ladders and steps. It may even be one of those pools that appear in oversized coffee-table books, with vanishing edges and spa extensions or wild, voluptuous curves.

Getting the cover size and shape right in that case takes a bit more savvy. Somehow the shape and measurement must be brought from the site to the scissors.

Cutting To Size

It’s quite a feat: A pool’s precise shape is communicated to a factory thousands of miles away.

And tie-down cover manufacturers admit that it can be daunting for a cover dealer to contemplate the measure of a pool whose shape resembles, say, a pork chop or the island of Oahu. “Often, pool-cover dealers are hesitant to measure a pool because they might make a mistake and then have to eat a cover that they measured improperly,” says Christensen.

If the job is simply beyond the dealer’s ken, adds Saltzman, the cover company can, in some cases, send a rep out to take the measurements. But that’s a rarity, as cover companies have step-by-step procedures which — if strictly followed — produce a perfect match at the factory. For free-form pools, many use a method called “AB measure,” which uses external reference points to orient a series of measurements.

The shape of the pool thus reduced to a chaotic jumble of figures, the task at the factory is to translate these numbers right back into the orginal shape. Often they are fed into an autoCAD system which builds a digital model of the pool. With a visual reference on screen, says Patel, “using our CAD-based system, we are able to determine if the measurements provided are correct.”

Anchor Industries saves a step in this process by putting CAD-type software directly into the hands of the cover dealer. “We provide a piece of software called ‘Coverworks’,” says Christensen, “that not only allows them to measure, but functions as a management tool that helps them determine where and when the measurement was done, and what orders are still pending or open.”

In either case, the primary goal has been achieved — that is, to produce a math-based model of the pool for the factory cutters to follow.

At some factories, workers study blueprints and then manually cut the fabric to the proper shape. In other cases, digital information from the CAD system is transferred directly to a computer-controlled cutting machine, which recreates the exact shape on the factory floor.

Straps And Webbing

With the cover now tailored to fit the pool, the means of holding it in place — webbing, straps and anchors — must be attached.

The webbing material itself is made of polypropylene, just like the mesh cover, but its tighter weave and distinctive finish give it a 4,000-pound break strength in tension. That’s not overkill, notes Saltzman, as the cover is exposed to enormous loads when in use from “heavy snows, large animals and humans.”

But the webbing material itself is not the usual point of failure. It’s more likely to be the point where the webbing is sewn to the cover. Close stitching with heavy thread make for durability here, notes LeeAnn Donaton, president, LOOPLOC, Ltd., Hauppauge, N.Y. “Stitching is a place where a cover can fail after years of use if not done with great care.”

Just as machine-gun ammunition sometimes includes white “tracer” bullets to help the gunner track the aim, LOOP-LOC feeds white thread into its sewing machines to mark where the needle has been. “That way, when it gets to inspection, if there’s a dropped stitch it will show up against the black material and we can correct it before it goes out.” While all manufacturers agree on the virtues of rugged and precise sewing, there is some divergence on the need for “double webbing” — meaning webbing sewn on the top and bottom of the cover.

Christensen, for one, points out that almost the entire load is borne by the webbing on the bottom, and that webbing on the top is completely superfluous. With the load underneath, he says, sewing the cover to the top webbing “is like nailing your flooring to the bottom of the floor joists.” Saltzman agrees that sewing on the cover should be kept to the minimum necessary. “Sewing top and bottom increases the likelihood of pinching the cover material and opening up the possibility of a tear, so it should be restricted only to areas where it is necessary to provide strength.”

His company’s policy includes double webbing, but sewn at the bottom only at the perimeter and within 4 feet of the edge of cover. With the cover load pushing the cover material down onto the lower straps, Saltzman says, further needlework is not necessary.

Patel’s Cover Company takes a third approach, and sews the top and bottom throughout. “Double webbing has two advantages,” she says. “Increased strength is the primary factor. Visual appearance is another. The double webbing throughout the cover keeps the webbing ‘lines’ straight upon installation.”

Finally, the springs are ready for attachment to the straps at the cover’s edge — the last step in assembly. The springs have a steel loop that hooks onto brass anchors drilled into the pool deck. By adjusting the straps, and using the tension in the springs, the cover installer can achieve that tight, barracks-bedsheet look. They’re simple contraptions, but springs are under a good deal of stress when in use, and play an important role in the cover’s function, notes Donaton.

“It is not common for a spring to fail or stretch out of shape. It can happen at times when the load on the cover is too great,” she says. But that’s OK, she adds, “It is intended to do just that.

“A spring that stretches out of shape in this condition actually helps take the stress off of the cover so the cover itself does not fail. It is much easier and less costly to replace a spring than to repair a custom-made cover.”

Such failures can be minimized by using high-grade domestic sources for hardware, according to a number of manufacturers, who advertise their spring gauge and strength as a point of differentiation.

With these firmly affixed, the cover is ready to be shipped and installed. Mesh or vinyl, there it lays, stretching from deck to deck — a forbidding barrier to falling leaves and children; its long, strange trip from the Jurassic plains to a customer’s backyard at an end.

Scott Webb is Executive Editor of AQUA Magazine.