How to Replace an Ink Sac?
By Edgar – On October 17th, 2016
Last revised on 3 November 2020
1Ink sacs – also called pen sacs – served as a reservoir for the ink of the fountain pen and were used in many of the writing instruments produced in the past. The most popular are probably the robust and widely produced lever fillers, but also other self-filling pens such as (push) button and twist fillers used conventional ink sacs. They were and still are made primarily of black natural rubber (latex), occasionally of soft PVC (vinyl) and in recent years also of transparent silicone.
I. Basic Information
2Natural Rubber — Ink sacs made of latex are usually the means of choice. They are very flexible and have a gas density which is sufficient for this purpose1. However, certain (modern) inks are capable of decomposing the material2, so problematic inks should not be used. Getting the sticky stuff out of the barrel is really no fun:
In addition, the material is suspected of being responsible for the more or less typical discoloration of old colored celluloid pens because it releases sulfur over time3. However, the ink residues that are released over the years when the ink sac decomposes could also be responsible. While the composition of latex ink sacs has changed after the advent of the first celluloid pens, the largest manufacturer claims to have received no reports of discolored celluloid pens in the last 30 years.4
3Silicone — They are transparent and can be used with transparent fountain pens where black ink sacs could disturb the appearance. The material is also chemically more stable, making them more durable and less susceptible to aggressive inks. On the other hand, they are not as flexible and their main disadvantage is that they have a relatively high gas permeability, so that some pens get rid of their ink within a few days depending on how the pen is stored. Fountain pens with a filled silicone ink sac must therefore always be stored upright (the nib points upwards).
4Soft PVC — They can be either transparent or black. The PVC used contains plasticizers that can outgas and cause massive damage to adjacent material. Ink sacs made of soft PVC should therefore only be used in fountain pens that are designed for this purpose (like the Aerometric Parker 515).
In the past, silicone ink sacs were sold that were actually made of soft PVC. When buying an ink sac, you have to pay attention to how the ink sac looks like, as they differ in the manufacturing process.6
2) Common Shapes
Continuously straight Necked Sac
3) Calculating the Size
7The sizes refer to the outer diameter of the ink sac in inches, which is the x-fold of 1/64. Thus, an ink sac with a size of 16 has a diameter of 16/64 inches:
8Size / 64 ≈ 0,25 inch × 25,4 = 6,35 millimeter
9The length of ink sacs that are continuously straight (Straight) does not have to be measured beforehand, but can easily be shortened to the correct length later.
10Ink sacs with a bottle-necked opening, which can be used in fountain pens with leverless systems, have a harder time of it, since they can only be shortened to a certain extent without changing their shape. So you have to research in advance which sac is the right one.
1) Separation of Section and Barrel
11In the past, manufacturers often sealed the grip section and barrel with a shellac solution. This effectively prevented the user from unscrewing or pulling of the grip section (even inadvertently). At the same time, it also served as protection against ink penetration if the fountain pen was inserted a little too deep into the ink bottle.7 Unfortunately, this strong connection between the grip section and the barrel also led to some (unsuccessful) opening experiments, because the parts simply could not be separated from each other by hand.
12The melting point of shellac is given differently and is roughly at a temperature in the range of 60 to 100 degrees8. In order to separate the grip section from the barrel without damage, the shellac, which is hard and brittle at room temperature, must be heated. There are several methods suitable for this, but only one is sufficiently safe to use, namely hot air. The temperature should not exceed 70 degrees, especially with sensitive materials like celluloid. Ebonite behaves less critically here because it neither deforms9 nor melts (without pressure). Heating is not only meant to soften the shellac, but also to reduce the risk of breaking the material.
13By the way, the grip section and the barrel were usually connected by a slip or friction fit, later also by a threaded fit. When opening, the section should therefore always be turned counterclockwise to avoid damage to any thread that may be present.
b) Loosen the Sealing
14The methods described have in common that they can fail on the first attempt. Sometimes the steps have to be repeated several times and it can even take days until it finally works.
15In order to have a better grip when unscrewing later, you should have a rubber glove ready. For large fountain pens the use of special pliers (Section Pliers) can be helpful. However, if the grip section is too small, as is the case with many of the fountain pens manufactured in the past, it can be squashed and damaged, so that in case of doubt it should be avoided to use pliers.
16Hot Air Device — Heat guns can cause not only the shellac connection to dissolve in a few seconds, but also the fountain pen itself, if operated without practice or set incorrectly. Celluloid is also highly flammable10. It is true that hot air guns are used by professional restorers because they can ensure that no critical temperatures are reached. In most cases, however, such devices are not necessary, so their use is not worthwhile for beginners anyway.
17Hairdryer — The use of a standard hairdryer is more recommendable. Switch this to the highest temperature setting and aim it at the area to be heated while turning the fountain pen all the time to avoid uneven heating. In doing so, you always try to unscrew the grip section from the barrel.
18Hot Water (Not recommended) — An effective but risky option is to dissolve the compound indirectly with hot water. This requires a pressure seal bag and a glass of boiling hot water. You put the tip of the fountain pen into the pressure seal bag. Then you dip the whole thing into the hot water for a few seconds, take it out again, dry it if necessary and immediately try to unscrew the grip section.
19The risk with this method is not so much the temperature itself – although it can be quite high, depending on how long you wait after pouring the water (the thin plastic bag is not able to reduce the temperature) – but the fact that the hardly heat-resistant11 pressure seal bag could leak during the procedure (or is from the beginning), which would cause the fountain pen to accidentally come into contact with the hot water. Celluloid can be damaged, which results in a milky-dull surface (clouding). Ebonite is not damaged by hot water, but water, whether hot or cold12, can cause unattractive green discoloration that was not previously visible. So if you did not intend to tediously restore these damages later on, but simply want to replace the ink sac, you should refrain from this method.
2) Removing the Old Ink Sac
20Before you insert a new ink sac, you need of course to remove the remains of the old ink sac. Sometimes these can be emptied from the barrel of the fountain pen just by tapping it lightly:
21The use of a screw has proven to be effective for removing stuck parts. Often it is possible to get out whole parts of the ink sac, especially if they stick to the sides of the barrel:
22Now the connecting piece of the grip section must also be refurbished. To be on the safe side, the feed should not be removed, as this would unnecessarily weaken the material of the grip section:
23The often rock-hard residues of the ink sac can splinter off and fly in all directions when the connector of the section is refurbished, so it is advisable to wear protective goggles.
3) Measuring the Size of the New Ink Sac
24Lever-operated fountain pens usually use a continuous straight ink bag (Straight).
If you don't have ink sacs, a good guide for determining the appropriate size is the diameter of the connector of the grip section of the fountain pen:
25The diameter of the shown connector is slightly more than 6 millimeters, so that you can choose between the sizes 15 (Ø 5.95 mm) and 16 (Ø 6.35 mm). However, you do not automatically have to choose the smaller version of the two, because the inner diameter of the ink sac is (about 1 mm) smaller than its outer diameter. If the ink sac does not sit loosely on the connector, but has a certain hold there, this is sufficient to be able to glue it properly later.
26However, if the untreated ink sac does not slide into the barrel of the fountain pen without pressure, it is too large, as it will later need space to expand due to temperature fluctuations. If in doubt, a smaller ink sac should be used, even if this reduces the usable ink volume.
4) Cutting the Length
27The ink sac must not be squeezed when the barrel and grip section are reconnected. To determine the correct length, you must first push the ink sac to the end of the barrel:
28Then simply cut off the protruding piece – but not too close to the thread to avoid damage:
29Now the ink sac is shortened again by about one centimeter or by the length of the connector:
5) Gluing the Ink Sac
30Ink sacs made of natural rubber are traditionally glued with a solution of shellac. With the help of a toothpick or a match, this adhesive can be easily applied:
31Once you have applied the shellac, you must promptly slip the ink sac over the connector and push it so that it sits straight. This process can be a bit difficult, but once the fresh shellac has been applied, it is usually easy to do.
32When you have glued the ink sac, the shellac still has to dry. Normally half an hour of waiting is enough to achieve a reasonably strong bond. You should not use the filling mechanism too much if you want to reconnect the grip section and the barrel soon.
33There is no alternative to shellac as an adhesive with few exceptions. The use of nail varnish or similar can lead to material damage and make later damage-free maintenance more difficult.
6) Talcum or Graphite
34The penultimate step is to coat the ink sac with talcum or graphite. Otherwise, the ink sac would wear out prematurely due to the frictional forces generated when operating the filling mechanism:
35Excess talcum can be shaken off, usually a moderate amount is sufficient:
36There is currently controversy in the scientific community as to whether talcum itself is carcinogenic – far from any (former) contamination with asbestos.13 In any case, the powder should not be inhaled – this applies to all dusts. If you are unsure about this, you can also use graphite14 as a substitute.
7) Reconnecting Grip Section and Barrel
a) Plugging or Screwing Together
37Screwed connections — The application example (see Measuring the Size of the New Ink Sac) is a fountain pen from the late 1940s, in which the grip section and barrel already had a screw connection. In this case it is sufficient to slowly screw the two parts back together again, taking care that the ink sac does not twist inside the barrel.
38Plugged connections — Much more frequently, however, plug connections are encountered. For optical and ergonomic reasons, it is important to ensure that the lever and nib are in a straight line when they are plugged together:
39 If the construction can only be plugged or screwed back together with increased force, the barrel must be heated so that it is not damaged.
40If the connection turns out to be a bit too loose later on, it can be fixed by applying a moderate amount of shellac to the thread or tenon of the grip section:
41It is not a malpractice to reunite the two parts in this way due to the advantages mentioned in a) Sealing after completion of the work. However, this is not absolutely necessary according to the prevailing opinion, especially not if the fountain pen remains in the hands of an expert or collector anyway. In addition, it must be taken into account that a too strong seal is always accompanied by the risk that the parts may not be able to be separated from each other without damage later on.