Pastes: The Practical
8 minute read
What you will learn
- Define a paste and say how it differs from an ointment
- List the uses of pastes
- Make a paste by mixing powders into a base
- Scale a paste formula
- Pack, label and store a paste
- Recognise faults in a paste
Key points
- A paste is a stiff semi-solid with a large amount of fine powder, usually 25% or more, in an ointment base.
- Pastes are stiffer and less greasy than ointments, and stay in place.
- They absorb fluid, protect the skin, and are used on small areas, such as for napkin rash or eczema.
- Powders must be fine and well mixed with the base.
- A paste is made by adding the powders to the softened base on a tile, little by little.
- Pastes are packed in wide-mouthed jars, and applied with a spatula.
- They are for external use only, and are not used on hairy areas.
What is a paste?
A paste is a stiff semi-solid that contains a large proportion of finely powdered solids, usually 25% or more, spread in a greasy base (or sometimes a water-based one). It is firmer than an ointment.
| Ointment | Paste | |
|---|---|---|
| Powder content | None, or small | Large (about 25% or more) |
| Texture | Soft and greasy | Stiff and less greasy |
| Spreading | Spreads easily | Stays where it is put |
| Absorbs fluid | Little | Absorbs secretions |
| Use | Soften, protect, carry drugs | Protect, absorb, treat small patches |
Uses
- To protect the skin, for example in napkin rash.
- To absorb moisture and discharges.
- To treat small, well-defined areas of skin disease such as eczema or psoriasis, because the paste stays on the patch and does not spread.
- To act as a barrier against irritants.
A paste is not used on hairy areas, because it is hard to remove, and not on large areas or weeping wounds unless directed.
Example: zinc and salicylic acid paste
Typical formula: salicylic acid 2%, zinc oxide 24%, starch 24%, white soft paraffin 50%. Use the formula your school gives you.
Scaling
Make 50 g.
- Salicylic acid = 50 × 2 ÷ 100 = 1 g
- Zinc oxide = 50 × 24 ÷ 100 = 12 g
- Starch = 50 × 24 ÷ 100 = 12 g
- White soft paraffin = 50 × 50 ÷ 100 = 25 g
Check. 1 + 12 + 12 + 25 = 50 g. The percentages add to 100%: 2 + 24 + 24 + 50 = 100.
Method
- Weigh all the ingredients.
- Sieve or powder the solids very finely, so there is no grit.
- Mix the powders together on a tile with a spatula, starting with the smallest quantity (the salicylic acid) and adding the larger ones (geometric dilution).
- Soften the base by working it with the spatula, or by warming it very gently if it is too stiff. Do not melt it unless the formula says.
- Add the powder to the base in portions, working each portion in thoroughly until the paste is smooth and even.
- Pack into a wide-mouthed jar, pressing out air gaps.
- Label the jar.
Why add the powder to the base?
Adding the powder to the base in stages wets each particle properly and gives a smooth, uniform paste. Adding all the base to all the powder at once makes lumps and gritty patches.
Packing, labelling and storage
- Pack in a wide-mouthed jar with a tight lid, because the paste is stiff and hard to remove from a tube.
- Label: patient's name, name and strength of the paste, directions, "For external use only", the date, and a discard date.
- Store in a cool place.
- Tell the patient: wash hands before and after use, apply with a clean spatula or gloved finger, and use a thin layer. Do not put it near the eyes or on broken skin unless told to do so.
- Salicylic acid can irritate healthy skin around the patch, so apply it only to the affected area.
Faults and what to do
| Fault | Likely cause | What to do |
|---|---|---|
| Gritty paste | The powders were too coarse, or not sieved | Powder and sieve them again |
| Lumps | Powder added all at once | Add in portions, mixing well |
| Paste too stiff | Too much powder, or a cold base | Check the weights, and warm the base very gently |
| Paste too soft | Too little powder or too much base | Check the weights |
| Uneven colour | Poor mixing | Continue mixing until smooth |
Exam focus
- State how a paste differs from an ointment.
- Describe the method for making a paste.
- Scale a paste formula, such as zinc and salicylic acid paste.
- Common mistake: using powders that are too coarse, which makes a gritty paste that irritates the skin.
Quick check
Test yourself before you move on. Pick an answer to see the explanation.
1.A paste differs from an ointment because it:
2.Zinc and salicylic acid paste has zinc oxide 24%, starch 24%, salicylic acid 2% and white soft paraffin 50%. How much white soft paraffin is in 50 g?
3.In the same paste, how much zinc oxide is in 50 g?
4.Why should powders for a paste be made very fine?
5.A paste is best applied with: