Ceramic Pottery Crawling & Bare Patches
Bald or Bare Patches On Fired Wares
When your glazed wares have patches of bare, raw or bald bisque this glaze defect is known as "crawling". However, please note that some products like creams, oils, baby wipes, grease and even pencils (with a wax content) can prevent the glaze from adhering and this can look very similar to crawling.
Make certain that bisqueware is completely clean and dry before applying underglaze and glaze.
Other measures include:
- Use thinner coats of underglaze and glaze. Try finger sanding heavy applications of underglaze or glaze prior to firing.
- Make sure the viscosity of the glaze is checked each time prior to dipping or brushing. During hot weather and when dipping lots of items, water needs to be added more frequently.
- Ensure the hole in the viscosity cup is cleaned with a cocktail stick each time it is used. The glaze can dry inside the hole thereby reducing the hole size and affecting the timings (dry glaze is barely visible in the hole so this exercise MUST be done).
- Make sure the items are not submerged below the glaze for too long – 4 secs maximum based on a viscosity reading of 19-24 seconds (all glaze viscosities vary - these are the timings for our clear glaze).
- If the dry glaze coat cracks or has holes/bubbles, rub it gently to fill it in.
- Allow coats of underglaze to dry between applications.
- Ensure bisqueware is not contaminated and clean prior to decorating.
- Avoid using products that may repel glaze and underglaze e.g. some pencils leave a waxy residue, baby wipes contain lanolin - all of which prevent the glaze and underglaze from adhering. Ideally hands should be cleaned with soap and water to remove grease, creams etc.
- Make sure the item is fully dry before glazing.
- Make sure the item is fully dry before firing.
- Try sponging your pieces before decorating. Some businesses claim sponging helps prevent crawling.
To salvage the piece try:
- Applying additional underglaze and glaze to the bare spot and re-fire. Remember that larger or thicker wares may be prone to cracking if re-fired so adjust the ramp settings for a slower fire and make sure the items are raised off the kiln shelf during firing.
- Make sure the glaze and underglaze application is about the same thickness as the original otherwise the final finish may not be smooth.
- On bad areas, try using a Dremel to file/flatten the surrounding area and then reapply colour and glaze.
Orton Ceramics have produced PDFs on Firing Tips. Click here to read more.