Q. Is this a Muslin Brush?

Is this a Muslin Brush?
Annie William using a 'wash' brush of some sort

This post is about a puzzle - about a brush.

I was asked yesterday whether Annie Williams was using a Muslin brush to lay down her unifying wash on the background of her painting. (see Annie Williams demonstrates a still life in watercolour)

I'd never heard of the term before and had to look it up.  I also looked to see whether I had a better photo of the brush - and above is the best I've got.

The wash brush that Annie William was using looks to me to be:

• longer than the length of a large hand with fingers stretched out.  
• The handle is very flat as is the ferrule around the hair.  
• The ferrule seems to be about the width of two large fingers. 
• The hairs used for the brush are very long - suggesting they come from a larger animal (ox?) and 
• the body of hair seemed to also be flattish and to taper into a filbert shape.  
• The hair worked well with water and seemed to be soft rather than stiff (ie a brush for watercolours not oils).

I immediately thought it might be a chinese brush - maybe a relative of the hake - but with longer hair.  It's like a flatter version of a mop brush.  The handle - which is very similar to that of a hake - made me wonder if it was some special sort of decorating brush.  I also wondered if it might be a lacquer brush.

The muslin brush

I had to look up muslin brush.  It appears that "muslin brush" is a term which has most of the search engines struggling!  I still don't know what it's for or why it has that name.

Below are the links I found to muslin brushes.  It would seem that ox hair seems to be the hair of choice.  Both brushes look squarer than this one which appeared to me to be rather more oval in shape.

• Muslin Brushes by Jerry's Artarama This is the description

These robust ox hair super thick flats can really move color on the painting surface. Oxhair is more absorbent than hog hair bristle but not as soft as taklon, sable or squirrel hair. An ox hair brush such as this should be in your assortment of brushes. They provide sure strokes on their wide or narrow side. Welded golden color nickel plated steel ferrules and short black handles.

• Magic Muslin brush from Cheap Joe's

Cheap Joe’s Magic Muslin watercolor paintbrush is entirely composed of natural ox hair. This feature along means that this brush has built-in durability, endurance, and a resilience that you won't believe! This art brush is a long and full shaped flat artists paint brush that performs as well on the wide side as it does on its narrow end

Does anybody know what a muslin brush is and why it has that name?