How Denim Is Made: Fabric Finishes

Discover Why Almost All Denim Is ‘Finished’ and Learn What Each of the Fabric Finishes Does

Fabric finishing is the last stage in the production of denim. It’s where the final touch is added, and it can make a big difference in how the fabric looks, feels and wears.

If you sell denim and jeans, being able to distinguish and explain the benefits of the most important fabric finishes to your customers is not just a nice-to-have. It’s vital information that can make you appear knowledgeable and trustworthy in sales and customer service situations, and it can prevent returns and customer complaints about unsanforized jeans shrink, legs that twist or coatings that wash out with the laundry water.

This final episode of my “how denim is made” series discusses how the two kinds of fabric finishes impact the way denim looks and behaves. The trick is to use this knowledge to your advance a make it part of the story you tell.

The five production stages of denim that the series discusses:

When I Realised That Most Denim Is ‘Finished’

When I started learning about denim about ten years ago, I didn’t pay much attention to fabric finishes. It was raw denim I wanted to know about, not the washed-down kind! I was obviously confused.

Most raw denim jeans are in fact treated one way or the other. Actually, these days, few jeans are sold as ‘loomstate,’ which is what ‘unfinished’ denim — in other words, untreated denim — is known as. Sure, loomstate denim used to be the only option available, but that was almost a century ago!

Hairy denim singeing

Without any finishing, denim looks “hairy” because of all the excess cotton fibres that sit on its surface. The singeing process takes care of the “problem.” What’s much more important is that loomstate denim doesn’t have any dimensional stability. That means it shrinks like crazy.

Even though shrinkage didn’t stop denim jeans from becoming a major hit, denim makers figured they could make the experience of buying and wearing denim jeans more enjoyable by taking some of the guesswork out of it. So they developed fabric finishes.

What Is Fabric Finishing?

Simply put, a fabric finish is a treatment given to change the appearance, touch or performance of a fabric. The goal is to enhance the fabric beyond its natural state to make it better suited for whatever its purpose is.

Technically, fabric finishes are divided into two categories: functional and aesthetic. They either work to correct and prevent issues of dimensional stability, or to enhance the look and feel of a fabric with what’s known as creative finishes. The processes can be mechanical, thermal or chemical.

Let’s look at the finishes almost all denim is treated with.

Functional Fabric Finishes

Fabric finishes that control dimensions are crucial because unfinished denim shrinks as much as 10%. The reason for that is summed up in one word: tension.

During spinning, dyeing and weaving, denim stretches in length and width as the yarn is under continuous tension. If the fibres are not shrunk back into their original size before the denim is made into a pair of jeans, they’ll shrink once washed as the fibres relax and return to their regular length.

Pre-Shrinking Helps You Find the Right Size

‘Pre-shrinking,’ also known as ‘sanforization,’ solves a fundamental problem of unfinished denim: it eliminates shrinkage. It makes buying a right-sized pair of jeans a whole lot easier. If you’ve ever tried shrink-to-fits, you know what I’m talking about.

That’s why sanforization is the most important fabric finish; hardly any jeans are completely unsanforized. In fact, according to former Levi’s fabric designer, Stefano Aldighieri of Another Design Studio, even Levi’s 501 shrink-to-fit jeans have some degree of sanforization (and pre-skewing) applied.

The beauty of unsanforized jeans is that they mould to your body in a way sanforized jeans simply won’t. What you get is a truly personalised fit. For me, there’s also something very special about the act itself of shrinking your jeans. I’ll get my little notebook out and measure my jeans before and after I shrink them. It’s mostly useful information, but then again I’m probably a junkie of that.

Indigofera Clint Shrink-to-Prima-Fit before and after
Indigofera Clint shrink-to-fit before and after shrinkage.

How Sanforization Works

Sanforization was invented in the late 1920s by American businessman and inventor, Sanford L. Cluett. The invention was a mechanical compaction process for woven fabrics.

The fabric is fed in between two hot roll bars on top of a stretchy endless rubber belt. The fabric is first moistened, usually by steam. This lubricates the fibres and primes shrinkability.

As the rubber returns to its original length once through the cylinders, the warp yarns shrink and the filling yarns are packed closer together. After the fabric leaves the rubber belt, it enters a dryer, which locks the fibres in their shrunken state.

Without sanforization, jeans might not have become the global phenomenon it is today, simply because a lot of consumers wouldn’t want to go through the trouble of shrinking their jeans themselves. Then again, it’s not only denim that’s sanforized. Virtually any woven fabric these days is pre-shrunk. Nevertheless, it’s an important fabric finish that makes jeans shopping a lot easier.

Sanforization process

But sanforization isn’t the only dimensional stability finish that most jeans undergo to make the experience more enjoyable for wearers.

Pre-Skewing Eliminates Leg Twist

It’s not only in length and width that the dimensions of unfinished denim change. They also change diagonally. Once unfinished denim is washed and shrinks, it naturally skews in the direction of the twill line.

The diagonal skew causes the infamous leg twist, which becomes progressively more noticeable when you wash your jeans as the angular relationship between the yarns relaxes. It’s one of the characteristics of vintage jeans, and it’s become a prized one. Back in the days, though, it was seen as a drawback.

The leg twist was a big issue for jeans makers in the 1960s. Consumers were returning their twisting jeans. The fact that the increasingly popular flared jeans accentuated the problem didn’t help.

In the mid-1960s, Wrangler introduced their solution to the problem: broken-twill, a denim with a zigzag twill pattern known to neutralize skewing in weaves like herringbone.

Levi’s, on the other hand, didn’t want to abandon their popular 3x1 twill. In the early 1970s, they put Karin Hakanson on the task of finding a solution. She invented the pre-skewing fabric finish that corrects the natural skew, and patented it in 1976 with Levi’s as the assignee.

Leg twist Momotaro jeans
Here’s what leg twist like on a pair of Momotaro jeans.

How Pre-Skewing Works

Remember, unfinished denim twists in the direction of the twill line. That means right-hand twill twists counterclockwise while left-hand twill twists clockwise.

The pre-skewing finish compensates for the tensions that cause the twist by deliberately skewing the denim in the opposite direction of the natural twist. That means right-hand twills are skewed counterclockwise and left-hand twills are skewed clockwise.

The weight, twill weave, yarn size, and yarn twist determine how much skewing is needing, but it’s usually between 4% and 10% for the denim in your jeans.

However, even though denim’s been skewed, you might still get leg twist on our raw denim jeans, only in the opposite direction of the twill line. That’s because the skewing process “over twists” the fabric in the opposite direction of the natural twist. As raw denim jeans haven’t been washed, you’ll still is that counter-skew, at least until they’re washed.

“Bad patterns or sewing can also cause leg twist, even with pre-skewed fabric,” Stefano Aldighieri argues. The way a fabric is cut and sewn can also result in a skew, especially with legs that taper, which effectively means all jeans that aren’t straight leg — in other words, slim fits, skinny fits and anti-fits.

While denim’s tendency to twist has been a dreaded feature for many years — and still is to the majority of consumers — some aficionados (like yours truly) actually want the legs of their jeans to twist. I’ll admit it, we’re a weird bunch! Paul Trynka also did a great little discussion of the topic over on his Loomstate blog.

Now, sanforization and skewing are probably the two functional fabrics finishes that denimheads talk about the most. I’m guilty of this myself. But there’s another finish that influences denim’s dimension that’s just as important for an emerging kind of denim.

Heat Setting Controls Stretchability

Stretch denim is becoming ever more present. That goes for the world of heritage and selvedge denim too. And that’s why we need to talk about the heat setting fabric finish.

To make stretch denim you need a synthetic elastomer that’s spun into the yarn — traditionally only the weft. To control how much stretch the fabric has, and it’s crosswise shrinkage, you need heat setting.

How Heat Setting Works

In this process, the fabric runs through a number of heat chambers. The heat changes the molecular structure of the synthetic stretch fibres. This allows the denim maker to control the fabric’s elasticity.

An even temperature is essential. If the temperature fluctuates, you’ll get uneven heat setting. The actual temperature varies, but for 10 oz. stretch denim you’ll need a temperature between 160°C and 200°C.

Machine speed is also important. The speed that the fabric runs through the machine is directly linked with how many heat chambers the machine has. Usually, there are 8–12 chambers; the more you have, the faster you can run the machine, which results in quicker production.

The elastomer content of the fabric also plays an important role. Basically, more stretch equals more heat. And the longer the denim is exposed to the heat, the less it will stretch.

That’s how denim’s stabilised. Now let’s get creative.

Creative Fabric Finishes

Regular ol’ blue jeans aren’t enough these days. The market constantly demands new colours, softer surfaces and great hand feels. That’s why denim makers are continuously exploring new technologies to enhance the tactile and optical aspects of the fabric.

While a niche of enthusiasts want their denim as raw as possible, most consumers don’t mind if their jeans are a little softer or if they have a “special” look. And that look often comes from a creative fabric finish. Let’s explore some of the most common ones.

Singeing Makes the “Hairy” Surface Smooth

The surface of unfinished denim looks a little like the upper lip of a teenage boy. Most consumers don’t want that.

The singeing finish smoothes the surface by removing the excess cotton fibres that give unfinished denim it’s “hairy” look. This is done by running the fabric quickly over a flame one side at a time. Most denim — even most raw denim — goes through this finish. In the video below, you can see how it’s done.

The “hairy” surface of unsinged denim is something aficionados love; it’s another characteristic of vintage denim. But while singeing affects the look and feel of a denim, it doesn’t do much to the way it fades.

Mercerization Makes the Denim More Lustrous

Unfinished denim has a characteristic rough and uneven hand feel. Enthusiasts love it, but most consumers don’t.

The solution often used is the chemical creative finished mercerization, named after its inventor, John Mercer. In mercerization, the fabric is treated under tension with a concentrated solution of caustic soda.

The finish opens the fibres and gives them a rounder shape, which makes the surface of the fabric smoother and increases the fabric’s lustre or shine. The finish is also used to prepare fabrics for further processing like overdyeing and coating, as it also enables the fibres to take in more dye.

For denim’s where mercerization gives a look that’s too flat and lustrous, the flat finish known as caustification is used. It’s sort of an in-between mercerized and unfinished denim.

Overdyed denim Iron Heart SEXIH07111BK
Iron Heart’s SEXIH07111BK collaboration with Self Edge is overdyed.

Add Colour With Overdyeing and Coating

Another common way to spice up the look of a fabric is adding a colour or a coat to it.

In overdyeing, the fabric is run through a bath of dye, which is then fixed by a steamer. A popular kind of overdye for denim these days is black sulfur on top of a warp-dyed indigo denim.

Coatings are for instance used to get a leathery look and feel. There are two types of coating methods. In knife coating, the coating material is deposited on the surface of the fabric and then the excess is scraped off with a blade. In foam coating, the coating material is turned into a foam and then sprayed onto the fabric. For both methods, once applied, the coating is fixed with heat.

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At the end of the day, there’s hardly any denim that’s not finished one way or the other, whether it’s to enhance performance or aesthetics.

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Thanks to Muzammil Usmani of Indigo Textile for his input on heat setting and creative finishes.